If your OBD-II scanner displays P0173, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the fuel trim correction for Bank 2 is outside the normal operating range or cannot be properly controlled.
The generic OBD-II definition for P0173 is:
Fuel Trim Malfunction (Bank 2)
Unlike P0171 (System Too Lean) or P0172 (System Too Rich), which indicate the engine is operating too lean or too rich, P0173 means the PCM has determined that the fuel trim system itself is no longer functioning correctly on Bank 2.
The PCM continuously adjusts fuel delivery using Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) based on feedback from oxygen sensors and other engine management sensors. When those corrections exceed calibrated limits or become unstable, P0173 is stored.
Common causes include:
- Vacuum leaks
- Dirty or faulty Mass Air Flow (MAF) sensor
- Leaking fuel injectors
- Low fuel pressure
- High fuel pressure
- Exhaust leaks
- Faulty oxygen sensor
- Intake manifold leaks
- Fuel delivery problems
- PCM faults (rare)
Unlike P0170, which affects Bank 1, P0173 specifically applies to Bank 2, the cylinder bank opposite cylinder number one on V-type engines.
What Does the P0173 Code Mean?
Modern engine management systems constantly adjust fuel delivery to maintain the ideal air-fuel ratio.
For gasoline engines, that target ratio is approximately:
- 14.7 parts air to 1 part fuel
The PCM uses information from numerous sensors, including:
- Mass Air Flow (MAF) sensor
- Oxygen sensors
- Air/Fuel Ratio sensors
- MAP sensor
- Engine coolant temperature sensor
- Intake air temperature sensor
- Fuel pressure sensor
Using this information, the PCM makes continuous fuel corrections through STFT and LTFT.
When the PCM determines it can no longer properly control fuel trim on Bank 2, it stores P0173.
What Is Fuel Trim?
Fuel trim refers to the PCM’s ability to increase or decrease fuel delivery to maintain the correct air-fuel ratio.
Two primary fuel trim values exist:
Short-Term Fuel Trim (STFT)
Fast adjustments based on immediate oxygen sensor feedback.
Long-Term Fuel Trim (LTFT)
Adaptive corrections learned over time.
Together they compensate for:
- Engine wear
- Fuel quality
- Temperature changes
- Altitude
- Sensor aging
- Component wear
When these corrections exceed acceptable limits, diagnostic trouble codes such as P0173 may be stored.
Where Is Bank 2?
Bank 2 refers to:
The engine bank opposite Cylinder Number One.
On inline engines:
- Only one cylinder bank exists.
- P0173 generally does not apply.
On V6, V8, V10, and V12 engines:
- Bank 1 contains Cylinder #1.
- Bank 2 contains the opposite cylinder bank.
Always verify cylinder numbering using the manufacturer’s service information.
How the PCM Detects P0173
The PCM continuously monitors:
- STFT
- LTFT
- Oxygen sensor activity
- MAF airflow
- Engine load
- RPM
- Coolant temperature
- Intake air temperature
If fuel trim corrections become unstable, exceed allowable limits, or cannot compensate for operating conditions, the PCM stores P0173.
Most manufacturers require the condition to occur during multiple drive cycles before illuminating the Check Engine Light.
Why Fuel Trim Matters
Proper fuel trim control is essential for:
- Fuel economy
- Engine performance
- Smooth idle
- Low emissions
- Catalytic converter protection
- Cold starting
- Engine longevity
When fuel trims become unstable, the engine may operate too rich or too lean under various conditions.
Can a Vacuum Leak Cause P0173?
Yes.
Although P0173 is a fuel trim malfunction code, vacuum leaks remain one of the most common underlying causes.
Common leak locations include:
- Intake manifold gasket
- Vacuum hoses
- PCV system
- Brake booster hose
- Intake plenum
- Throttle body gasket
Unmetered air entering Bank 2 forces the PCM to make excessive fuel corrections.
Can a Dirty MAF Sensor Cause P0173?
Absolutely.
The MAF sensor measures incoming airflow.
If contamination causes inaccurate airflow readings, the PCM calculates incorrect fuel delivery.
This forces excessive fuel trim adjustments until P0173 is eventually stored.
Common contaminants include:
- Dirt
- Dust
- Oil from reusable air filters
- Moisture
Cleaning the MAF sensor often restores proper airflow calculations.
Can Fuel Pressure Problems Cause P0173?
Yes.
Both low and high fuel pressure affect fuel trim.
Possible causes include:
- Weak fuel pump
- Faulty fuel pressure regulator
- Restricted fuel filter
- Fuel return restrictions
- Incorrect aftermarket fuel pump
Any fuel delivery problem requiring excessive fuel correction may trigger P0173.
P0173 Quick Facts
| Item | Information |
|---|---|
| DTC | P0173 |
| Generic Definition | Fuel Trim Malfunction (Bank 2) |
| System | Fuel & Air Management |
| Severity | Moderate |
| Safe to Drive? | Usually, but repairs should not be delayed |
| Common Symptoms | Check Engine Light, rough idle, hesitation, poor fuel economy |
| Common Causes | Vacuum leaks, dirty MAF sensor, fuel pressure problems, injector faults |
| Typical Repair Cost | $100–1,500+ |
| Related Codes | P0170, P0171, P0172, P0174, P0175 |
Common Vehicles That Experience P0173
P0173 commonly appears on:
- Ford
- Chevrolet
- GMC
- Dodge
- Ram
- Toyota
- Lexus
- Nissan
- Infiniti
- BMW
- Mercedes-Benz
- Audi
- Volkswagen
- Hyundai
- Kia
It is most commonly found on V-type engines that monitor separate fuel trims for each cylinder bank.
What Does P0173 Really Mean?
The wording of P0173 often causes drivers to believe the fuel trim system itself has failed.
The code does not automatically mean:
- The PCM has failed.
- The oxygen sensor is defective.
- The fuel injectors are bad.
- The fuel pump has failed.
- The engine is damaged.
Instead, it means the PCM has determined that fuel trim corrections on Bank 2 have exceeded normal operating limits or cannot be properly controlled.
The underlying fault may involve:
- Vacuum leaks
- Fuel delivery
- Airflow measurement
- Oxygen sensor feedback
- Fuel injectors
- Intake leaks
- Exhaust leaks
- Engine mechanical problems
Think of P0173 as the PCM saying:
“I’ve reached the limit of my fuel correction strategy on Bank 2. Something is preventing me from maintaining the proper air-fuel mixture, and further diagnosis is required.”
Common P0173 Scan Tool Patterns
Monitoring live scan-tool data is one of the most effective ways to diagnose P0173.
Unlike simply reading the stored trouble code, fuel trim data reveals why the PCM can no longer maintain proper fuel correction on Bank 2.
Professional scan tools allow technicians to monitor:
- Short-Term Fuel Trim (STFT)
- Long-Term Fuel Trim (LTFT)
- Mass Air Flow (MAF) readings
- Oxygen sensor activity
- Air/Fuel Ratio (AFR) sensor data
- Fuel pressure
- Engine load
- Intake manifold pressure
- Engine coolant temperature
These values help determine whether the problem is caused by excess air, excessive fuel delivery, inaccurate sensor inputs, or mechanical faults.
Pattern 1: Excessively Positive Fuel Trims
The most common scan-tool pattern is:
- STFT: +18% to +30%
- LTFT: +12% to +25%
Possible causes include:
- Vacuum leak
- Intake manifold leak
- Low fuel pressure
- Dirty MAF sensor
- Restricted fuel injectors
Positive fuel trims indicate the PCM is continuously adding fuel to compensate for a lean condition.
Pattern 2: Excessively Negative Fuel Trims
Some vehicles display:
- STFT: –18% to –30%
- LTFT: –12% to –25%
Possible causes include:
- Leaking fuel injectors
- Excessive fuel pressure
- Stuck-open EVAP purge valve
- Dirty MAF sensor
- Faulty coolant temperature sensor
Negative fuel trims indicate the PCM is removing fuel to compensate for a rich condition.
Pattern 3: Fuel Trims Change Rapidly
Some scan tools show:
- STFT constantly swinging
- LTFT changing frequently
- Oxygen sensor switching excessively
Possible causes include:
- Intermittent vacuum leak
- Failing oxygen sensor
- Loose electrical connector
- Wiring fault
- Intake leak
Rapid fuel trim fluctuations usually indicate an unstable air-fuel mixture.
Pattern 4: Bank 2 Fuel Trims Differ Greatly From Bank 1
Compare:
- Bank 1 STFT
- Bank 2 STFT
- Bank 1 LTFT
- Bank 2 LTFT
If Bank 2 fuel trims differ dramatically from Bank 1:
Possible causes include:
- Bank 2 intake leak
- Bank 2 exhaust leak
- Bank 2 injector problem
- Bank 2 oxygen sensor issue
Comparing both banks is often one of the quickest ways to isolate the fault.
Pattern 5: Low MAF Airflow
Monitor airflow at idle.
Typical airflow values:
- 2.0L engine: 2–4 g/sec
- 3.5L V6: 4–7 g/sec
- 5.3L V8: 5–9 g/sec
Lower-than-expected airflow often indicates:
- Dirty MAF sensor
- Failing MAF sensor
- Restricted air intake
Incorrect airflow calculations force excessive fuel trim corrections.
Pattern 6: Fuel Pressure Outside Specifications
Measure fuel pressure.
Possible findings:
- Pressure too low
- Pressure too high
- Pressure unstable
Possible causes include:
- Weak fuel pump
- Fuel pressure regulator failure
- Restricted fuel filter
- Fuel return restriction
Fuel pressure problems frequently produce abnormal fuel trims.
Pattern 7: P0173 Appears With Other Fuel Trim Codes
P0173 commonly appears alongside:
- P0170
- P0171
- P0172
- P0174
- P0175
Multiple fuel trim codes often indicate:
- Dirty MAF sensor
- Fuel delivery problem
- Large vacuum leak
- Sensor failure affecting both banks
Common Symptoms of P0173
Symptoms vary depending on whether the fuel trim malfunction causes a rich or lean mixture.
Common symptoms include:
- Check Engine Light
- Rough idle
- Engine hesitation
- Poor acceleration
- Reduced fuel economy
- Hard starting
- Fuel odor
- Engine stumble
- Occasional misfires
- Failed emissions inspection
Some vehicles initially show only a Check Engine Light.
Check Engine Light
The Check Engine Light is usually the first symptom.
Most manufacturers require:
- Excessive fuel trim corrections
- Stable engine operating conditions
- Multiple failed self-tests
before storing P0173.
Rough Idle
Fuel trim problems often produce:
- Rough idle
- Idle speed fluctuations
- Slight engine vibration
- Occasional stalling
Idle quality frequently worsens before other symptoms appear.
Hesitation During Acceleration
Improper fuel trim calculations may cause:
- Delayed throttle response
- Flat spots
- Engine stumble
- Poor acceleration
The severity depends on how far fuel trims have moved from normal.
Reduced Fuel Economy
When fuel trims become unstable:
- Injector pulse width increases or decreases excessively.
- Fuel consumption often rises.
- Engine efficiency declines.
Poor fuel economy is a common complaint with P0173.
Is P0173 Serious?
P0173 is generally considered a moderate severity diagnostic trouble code.
Ignoring the problem may eventually lead to:
- Engine misfires
- Poor fuel economy
- Catalytic converter damage
- Increased emissions
- Carbon buildup
- Reduced engine performance
Repairs should be completed promptly to prevent additional damage.
Can You Drive With P0173?
Usually, yes—but repairs should not be delayed.
Driving is generally safe if:
- The engine runs normally.
- No flashing Check Engine Light is present.
- Severe drivability symptoms are absent.
Stop driving immediately if:
- The Check Engine Light flashes.
- Severe misfires develop.
- Engine power drops significantly.
- Heavy fuel odor or black smoke appears.
Prolonged operation with incorrect fuel trims may damage the catalytic converter.
P0173 vs Related Trouble Codes
Understanding related fuel trim codes simplifies diagnosis.
P0170
Fuel Trim Malfunction (Bank 1)
The PCM detects abnormal fuel trim corrections on Bank 1.
P0171
System Too Lean (Bank 1)
The PCM is adding excessive fuel to compensate for a lean mixture.
P0172
System Too Rich (Bank 1)
The PCM is removing excessive fuel because Bank 1 is running rich.
P0173
Fuel Trim Malfunction (Bank 2)
The PCM can no longer properly control fuel trim corrections on Bank 2.
P0174
System Too Lean (Bank 2)
Bank 2 requires excessive positive fuel trim.
P0175
System Too Rich (Bank 2)
Bank 2 requires excessive negative fuel trim.
30 Common Causes of P0173
The most common causes include:
- Vacuum leak on Bank 2
- Intake manifold gasket leak
- Dirty MAF sensor
- Failed MAF sensor
- Low fuel pressure
- High fuel pressure
- Weak fuel pump
- Restricted fuel filter
- Faulty fuel pressure regulator
- Leaking fuel injectors
- Restricted fuel injectors
- Exhaust leak before the oxygen sensor
- Faulty upstream oxygen sensor
- Faulty Air/Fuel Ratio sensor
- Stuck-open EVAP purge valve
- Faulty MAP sensor
- Faulty engine coolant temperature sensor
- Dirty throttle body
- Restricted air filter
- Intake duct leak
- PCV system leak
- Charging system problems
- Low battery voltage
- Wiring damage
- Poor electrical grounds
- Fuel contamination
- Incorrect aftermarket intake installation
- Engine mechanical problems
- PCM software issue
- PCM failure (rare)
Final Thoughts
The P0173 Fuel Trim Malfunction (Bank 2) code indicates the PCM has lost the ability to properly control fuel trim corrections on Bank 2. While the code points to a fuel trim malfunction, the underlying cause is usually vacuum leaks, dirty MAF sensors, fuel pressure problems, injector faults, or inaccurate sensor data rather than a failed PCM.
Proper diagnosis should determine whether the fault originates in:
- Air intake system
- Vacuum system
- Fuel delivery system
- Fuel injectors
- MAF sensor
- Oxygen sensors
- Engine mechanical condition
- PCM
The next section of this guide covers:
- Complete diagnostic procedures
- Fuel trim analysis
- Vacuum leak testing
- Fuel pressure testing
- MAF sensor testing
- Repair procedures
- Repair costs
- Manufacturer-specific troubleshooting
Following a structured diagnostic process prevents unnecessary parts replacement while restoring proper fuel trim control, engine performance, fuel economy, and emissions compliance.
Related Pro Street Diagnostic Guides
- P0170 – Fuel Trim Malfunction (Bank 1)
- P0171 – System Too Lean (Bank 1)
- P0172 – System Too Rich (Bank 1)
- P0174 – System Too Lean (Bank 2)
- P0175 – System Too Rich (Bank 2)
- P0101 – Mass Air Flow Sensor Range/Performance
How to Diagnose the P0173 Code
Diagnosing P0173 requires determining why the PCM can no longer properly control fuel trim on Bank 2.
Unlike P0174 (System Too Lean) or P0175 (System Too Rich), P0173 does not immediately indicate whether the engine is operating rich or lean. Instead, it indicates that the fuel correction strategy for Bank 2 has exceeded normal operating limits or has become unstable.
The underlying problem may involve:
- Vacuum leaks
- Intake leaks
- Dirty or faulty MAF sensor
- Low fuel pressure
- High fuel pressure
- Fuel injector problems
- Exhaust leaks
- Oxygen sensor faults
- Wiring problems
- Engine mechanical issues
Proper diagnosis should identify the root cause before replacing expensive components.
Safety Precautions
Before beginning diagnosis:
- Park the vehicle on level ground.
- Apply the parking brake.
- Wear safety glasses.
- Allow hot engine components to cool.
- Keep sparks and open flames away from fuel system components.
- Work in a well-ventilated area.
Fuel system testing involves pressurized gasoline and should always be performed carefully.
Tools Required
Professional diagnosis may require:
- Professional scan tool
- Live-data capability
- Fuel pressure gauge
- Smoke machine
- Digital multimeter
- Vacuum gauge
- Fuel injector balance tester
- MAF sensor cleaner
- Basic hand tools
- Manufacturer wiring diagrams
A professional scan tool capable of monitoring both Bank 1 and Bank 2 fuel trims simultaneously greatly simplifies diagnosis.
Step 1 – Verify the Code
Connect a professional scan tool.
Record:
- Stored DTCs
- Pending DTCs
- Permanent DTCs
- Freeze-frame data
Monitor:
- STFT Bank 1
- LTFT Bank 1
- STFT Bank 2
- LTFT Bank 2
- Engine load
- RPM
- MAF airflow
- Oxygen sensor activity
Always save freeze-frame data before clearing codes.
Step 2 – Check for Related Trouble Codes
P0173 commonly appears with:
- P0170
- P0171
- P0172
- P0174
- P0175
- P0101
- P0300
Related codes often simplify diagnosis.
| Code Combination | Likely Diagnosis |
|---|---|
| P0173 only | Bank 2 fuel trim problem |
| P0173 + P0174 | Lean condition affecting Bank 2 |
| P0173 + P0175 | Rich condition affecting Bank 2 |
| P0173 + P0101 | Dirty or faulty MAF sensor |
| P0173 + P0300 | Fuel trim issue causing engine misfires |
Repair related faults before replacing additional components.
Step 3 – Analyze Fuel Trim Data
Fuel trims are the single most important diagnostic tool for P0173.
Monitor:
- STFT Bank 2
- LTFT Bank 2
Normal values:
- ±5%
Acceptable:
- ±10%
Problematic:
- Above +15%
- Below –15%
Severe:
- Above +25%
- Below –25%
Compare Bank 2 values with Bank 1 to identify whether the problem is isolated or affects the entire engine.
Step 4 – Compare Fuel Trims Between Banks
Compare:
- STFT Bank 1
- LTFT Bank 1
- STFT Bank 2
- LTFT Bank 2
Large differences between the two banks often indicate:
- Intake leak on Bank 2
- Exhaust leak on Bank 2
- Fuel injector problem
- Oxygen sensor fault
- Mechanical issue affecting one cylinder bank
This comparison is one of the quickest ways to narrow the diagnosis.
Step 5 – Inspect the Air Intake System
Inspect:
- Air filter
- Air filter housing
- Intake duct
- Hose clamps
- Air intake tubing
Look for:
- Cracks
- Loose clamps
- Torn intake boots
- Dirty air filter
- Improper aftermarket intake installation
Unmetered air or restricted airflow can significantly alter fuel trim calculations.
Step 6 – Inspect for Vacuum Leaks
Inspect:
- Vacuum hoses
- Intake manifold gasket
- PCV hoses
- Brake booster hose
- EVAP vacuum lines
- Throttle body gasket
Use a smoke machine whenever possible.
Vacuum leaks affecting only Bank 2 commonly trigger P0173.
Step 7 – Inspect the Mass Air Flow (MAF) Sensor
Inspect for:
- Dirt
- Oil contamination
- Water intrusion
- Damaged sensing wires
- Loose electrical connector
Clean the sensor only with MAF sensor cleaner.
Never touch the sensing element.
Step 8 – Monitor MAF Airflow
Observe live airflow.
Typical idle airflow:
- 2.0L engine: 2–4 g/sec
- 3.5L V6: 4–7 g/sec
- 5.3L V8: 5–9 g/sec
Low airflow readings commonly indicate:
- Dirty MAF sensor
- Failed MAF sensor
- Restricted intake
Step 9 – Measure Fuel Pressure
Using a fuel pressure gauge:
Compare pressure with manufacturer specifications.
Improper fuel pressure may indicate:
- Weak fuel pump
- Faulty fuel pressure regulator
- Restricted fuel filter
- Fuel return restriction
Fuel delivery problems frequently create unstable fuel trim corrections.
Step 10 – Inspect Fuel Injectors
Inspect injectors for:
- Fuel leakage
- Restricted flow
- Electrical faults
- Uneven injector balance
Injector imbalance affecting Bank 2 commonly produces abnormal fuel trims.
Step 11 – Inspect the Upstream Oxygen Sensor
Monitor Bank 2 upstream oxygen sensor activity.
Verify:
- Proper switching
- Correct response time
- Stable voltage
- Heater circuit operation
A sluggish oxygen sensor may contribute to unstable fuel trim corrections.
Step 12 – Inspect for Exhaust Leaks
Inspect the exhaust system before the Bank 2 upstream oxygen sensor.
Look for:
- Cracked manifold
- Broken manifold studs
- Leaking gaskets
- Loose flanges
Fresh air entering the exhaust can falsely alter oxygen sensor readings.
Step 13 – Review Technical Service Bulletins
Before replacing expensive components:
Review manufacturer Technical Service Bulletins (TSBs).
Look for:
- PCM calibration updates
- Fuel trim strategy revisions
- MAF sensor updates
- Oxygen sensor bulletins
Many manufacturers have released software updates addressing false fuel trim codes.
Step 14 – Review Freeze-Frame Data
Compare current operating conditions with freeze-frame information.
Review:
- Engine load
- RPM
- Vehicle speed
- Fuel trims
- Coolant temperature
Freeze-frame data often reveals exactly when the PCM detected abnormal fuel trim behavior.
Step 15 – Inspect Wiring and Connectors
Inspect connectors for:
- MAF sensor
- Oxygen sensor
- Fuel injectors
- MAP sensor
Look for:
- Corrosion
- Bent terminals
- Loose pins
- Damaged wiring
Electrical faults frequently affect sensor accuracy.
Step 16 – Verify Charging System Voltage
Measure charging voltage.
Normal charging voltage:
13.5–14.8 volts
Low voltage can affect injector operation and sensor accuracy.
Step 17 – Inspect Previous Repairs
Inspect for:
- Incorrect aftermarket sensors
- Intake modifications
- Poor wiring repairs
- Fuel system modifications
- Incorrect vacuum hose routing
Many P0173 cases begin after unrelated repair work.
Step 18 – Evaluate Engine Mechanical Condition
Perform:
- Compression test
- Vacuum test
- Leak-down test (if necessary)
Mechanical problems affecting Bank 2 can force abnormal fuel trim corrections.
Step 19 – Perform a Road Test
Monitor live data during:
- Idle
- Cruise
- Moderate acceleration
- Heavy acceleration
Observe:
- STFT
- LTFT
- Oxygen sensor activity
- MAF airflow
Fuel trims should remain stable and close to normal throughout the drive cycle.
Step 20 – Confirm the Diagnosis
Before replacing major components, verify:
- Vacuum system integrity
- Air intake integrity
- MAF sensor accuracy
- Fuel pressure
- Injector balance
- Oxygen sensor operation
- Exhaust integrity
- Charging system voltage
Completing these diagnostic steps prevents unnecessary parts replacement while accurately identifying the root cause of P0173.
Step 21 – Repair Vacuum Leaks
If diagnosis confirms a vacuum leak on Bank 2, repairing the leak is often the quickest and most effective solution.
Inspect and repair:
- Vacuum hoses
- Intake manifold gasket
- PCV hoses
- Brake booster hose
- EVAP vacuum lines
- Throttle body gasket
- Intake plenum seals
After repairs:
- Clear all diagnostic trouble codes.
- Restart the engine.
- Monitor Bank 2 fuel trims.
Even a small vacuum leak can force the PCM to make large fuel trim corrections.
Step 22 – Clean or Replace the Mass Air Flow (MAF) Sensor
A contaminated MAF sensor commonly causes unstable fuel trim calculations.
General procedure:
- Turn the ignition off.
- Remove the MAF sensor.
- Spray the sensing element using MAF sensor cleaner only.
- Allow the sensor to dry completely.
- Reinstall the sensor.
- Clear all DTCs.
- Verify fuel trims during a road test.
If cleaning fails to restore accurate airflow readings, replace the sensor with an OEM-quality unit.
Using carburetor cleaner on a MAF sensor is a remarkably efficient way to create additional repair bills.
Step 23 – Repair Fuel Delivery Problems
Improper fuel delivery can cause excessive fuel trim corrections.
Inspect and repair:
- Weak fuel pump
- Restricted fuel filter
- Fuel pressure regulator
- Fuel pressure sensor
- Fuel return line restrictions
Always verify fuel pressure after repairs before replacing additional components.
Step 24 – Repair Fuel Injector Problems
Inspect injectors for:
- Fuel leakage
- Restricted flow
- Poor spray pattern
- Electrical faults
- Uneven injector balance
Repair or replace injectors that fail balance or leakage testing.
Injector problems affecting only Bank 2 commonly trigger P0173.
Step 25 – Repair Intake Air Leaks
Inspect and repair:
- Intake duct cracks
- Loose hose clamps
- Torn intake boots
- Damaged intake manifold
- Improper aftermarket intake installation
Any unmetered air entering Bank 2 forces excessive fuel trim corrections.
Step 26 – Repair Exhaust Leaks
Inspect the exhaust system before the Bank 2 upstream oxygen sensor.
Repair:
- Cracked exhaust manifolds
- Broken manifold studs
- Damaged exhaust gaskets
- Loose flange connections
- Cracked exhaust pipes
Fresh outside air entering ahead of the oxygen sensor may falsely alter fuel trim calculations.
Step 27 – Repair Oxygen Sensor or Wiring Problems
If previous testing indicates inaccurate oxygen sensor operation:
Inspect:
- Heater circuit
- Sensor wiring
- Connector condition
- Sensor response time
Replace the upstream oxygen sensor only after verifying:
- Proper fuel pressure
- Correct MAF readings
- No vacuum leaks
- Proper injector operation
The oxygen sensor often reports fuel trim problems rather than causing them.
Step 28 – Update PCM Software
Before replacing expensive components:
Review manufacturer Technical Service Bulletins (TSBs).
Manufacturers occasionally release software updates addressing:
- Fuel trim strategy
- Adaptive learning
- Oxygen sensor calibration
- MAF calibration
- False P0173 detection
Always verify the PCM has the latest available software.
Step 29 – Verify Fuel Trim Operation
After repairs:
Monitor:
- STFT Bank 2
- LTFT Bank 2
- Oxygen sensor activity
- MAF airflow
- Idle quality
Normal fuel trims should generally remain within:
- ±5%
Values consistently within ±10% are considered acceptable on most vehicles.
Compare Bank 2 with Bank 1 to ensure both cylinder banks operate similarly.
Step 30 – Consider PCM Failure
PCM failure is extremely uncommon.
Only suspect the PCM after confirming:
- No vacuum leaks
- Proper MAF operation
- Correct fuel pressure
- Proper injector balance
- Accurate oxygen sensor operation
- Intake integrity
- Exhaust integrity
- Charging system voltage
- PCM software updates
PCM replacement should always be the final diagnostic step.
How to Fix P0173
The proper repair depends entirely on the diagnostic results.
Common repairs include:
- Repairing vacuum leaks
- Cleaning or replacing the MAF sensor
- Repairing fuel pressure problems
- Replacing leaking fuel injectors
- Repairing intake leaks
- Repairing exhaust leaks
- Replacing a faulty oxygen sensor
- Repairing damaged wiring
- Updating PCM software
- Replacing the PCM (rare)
Verifying the Repair
After completing repairs:
- Clear all diagnostic trouble codes.
- Start the engine.
- Allow the engine to reach operating temperature.
- Monitor STFT and LTFT for both banks.
- Perform idle, cruise, and acceleration testing.
- Complete a full OBD-II drive cycle.
- Confirm no pending or permanent DTCs return.
- Perform a final scan to verify a successful repair.
A successful repair restores stable fuel trim operation, proper air-fuel ratio, smooth engine performance, improved fuel economy, and emissions compliance.
Typical Repair Costs
| Repair | Typical Cost |
|---|---|
| Professional diagnosis | $100–250 |
| Vacuum hose replacement | $25–200 |
| Intake manifold gasket replacement | $250–900 |
| MAF sensor cleaning | $15–40 |
| MAF sensor replacement | $150–500 |
| Fuel pressure test | $75–150 |
| Fuel pump replacement | $500–1,500 |
| Fuel pressure regulator replacement | $200–600 |
| Fuel injector cleaning | $100–300 |
| Fuel injector replacement | $150–600 each |
| Oxygen sensor replacement | $200–500 |
| Exhaust manifold repair | $300–1,200 |
| PCM software update | $100–250 |
| PCM replacement | $800–2,000+ |
Repair costs vary depending on vehicle design, labor rates, and the root cause of the fuel trim malfunction.
Common Diagnostic Mistakes
Avoid these common mistakes:
- Replacing the oxygen sensor before analyzing fuel trims
- Ignoring vacuum leaks on Bank 2
- Skipping MAF sensor cleaning
- Replacing injectors without balance testing
- Ignoring fuel pressure testing
- Clearing codes before recording freeze-frame data
- Ignoring Technical Service Bulletins
- Overlooking exhaust leaks ahead of the oxygen sensor
- Replacing the PCM before completing electrical and mechanical testing
Many P0173 repairs are resolved by correcting airflow or fuel delivery issues rather than replacing major components.
Manufacturer-Specific Notes
Ford
Inspect:
- Bank 2 vacuum leaks
- PCV system
- MAF sensor contamination
- Fuel pressure
Ford engines commonly develop intake leaks that affect one cylinder bank.
General Motors (Chevrolet/GMC)
Verify:
- MAF sensor readings
- Fuel pressure
- Injector balance
- Intake manifold sealing
Fuel trim malfunctions are commonly traced to airflow measurement or injector performance.
Toyota/Lexus
Inspect:
- Intake manifold gasket
- MAF sensor
- Fuel trims
- Oxygen sensor operation
Toyota engines often respond well to MAF cleaning and intake leak repairs.
Honda/Acura
Inspect:
- Vacuum hoses
- Fuel injectors
- PCV system
- Intake manifold
Vacuum leaks and injector restrictions frequently cause abnormal fuel trims.
Nissan/Infiniti
Verify:
- Intake ducts
- MAF sensor
- Fuel pressure
- Oxygen sensor operation
Cracked intake tubing and contaminated MAF sensors are common causes.
BMW/Mercedes-Benz
Inspect:
- Crankcase ventilation system
- Intake boots
- Vacuum leaks
- Fuel trim values
European engines frequently require smoke testing to locate small intake leaks.
Hyundai/Kia
Inspect:
- MAF sensor
- Intake manifold gasket
- Fuel pressure
- Injector balance
Airflow measurement problems are a frequent contributor to P0173.
Frequently Asked Questions
What usually fixes P0173?
The most common repairs include fixing vacuum leaks, cleaning or replacing the MAF sensor, repairing fuel pressure problems, replacing leaking injectors, repairing intake or exhaust leaks, or updating PCM software.
Can I drive with P0173?
Usually, yes. However, prolonged operation with unstable fuel trims can reduce fuel economy, increase emissions, cause poor performance, and eventually damage the catalytic converter. Repairs should be completed promptly.
Does P0173 always mean the oxygen sensor is bad?
No. In most cases, the oxygen sensor is accurately reporting abnormal combustion conditions. Vacuum leaks, airflow problems, fuel delivery issues, and injector faults are much more common causes.
Can a vacuum leak cause P0173?
Yes. A vacuum leak affecting Bank 2 forces the PCM to continuously adjust fuel delivery, eventually exceeding its adaptive correction limits.
Can a dirty MAF sensor cause P0173?
Absolutely. A contaminated MAF sensor provides inaccurate airflow information, forcing the PCM to make excessive fuel trim corrections until P0173 is stored.
Is P0173 a serious code?
P0173 is generally considered a moderate severity diagnostic trouble code. If ignored, it can lead to poor fuel economy, drivability issues, increased emissions, catalytic converter damage, and additional fuel system faults.
Final Thoughts
The P0173 Fuel Trim Malfunction (Bank 2) code indicates the PCM has reached the limit of its ability to properly control fuel trim corrections for Bank 2. While the code points to a fuel trim malfunction, the underlying cause is usually vacuum leaks, dirty MAF sensors, fuel pressure problems, injector issues, intake leaks, or inaccurate sensor inputs rather than PCM failure.
A complete diagnosis should always verify:
- Fuel trim data
- Vacuum system integrity
- MAF sensor accuracy
- Fuel pressure
- Injector balance
- Oxygen sensor operation
- Intake and exhaust integrity
- PCM software level
Following a systematic diagnostic process prevents unnecessary parts replacement while restoring stable fuel trim operation, engine performance, fuel economy, emissions compliance, and long-term reliability.
Continue Your Pro Street Fuel System Series
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