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P0173 Code Explained: Fuel Trim Malfunction (Bank 2) Causes, Symptoms & Fixes

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:

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:

The PCM uses information from numerous sensors, including:

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:

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:

On V6, V8, V10, and V12 engines:

Always verify cylinder numbering using the manufacturer’s service information.

How the PCM Detects P0173

The PCM continuously monitors:

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:

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:

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:

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:

Any fuel delivery problem requiring excessive fuel correction may trigger P0173.

P0173 Quick Facts

ItemInformation
DTCP0173
Generic DefinitionFuel Trim Malfunction (Bank 2)
SystemFuel & Air Management
SeverityModerate
Safe to Drive?Usually, but repairs should not be delayed
Common SymptomsCheck Engine Light, rough idle, hesitation, poor fuel economy
Common CausesVacuum leaks, dirty MAF sensor, fuel pressure problems, injector faults
Typical Repair Cost$100–1,500+
Related CodesP0170, P0171, P0172, P0174, P0175

Common Vehicles That Experience P0173

P0173 commonly appears on:

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:

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:

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:

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:

Possible causes include:

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:

Possible causes include:

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:

Possible causes include:

Rapid fuel trim fluctuations usually indicate an unstable air-fuel mixture.

Pattern 4: Bank 2 Fuel Trims Differ Greatly From Bank 1

Compare:

If Bank 2 fuel trims differ dramatically from Bank 1:

Possible causes include:

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:

Lower-than-expected airflow often indicates:

Incorrect airflow calculations force excessive fuel trim corrections.

Pattern 6: Fuel Pressure Outside Specifications

Measure fuel pressure.

Possible findings:

Possible causes include:

Fuel pressure problems frequently produce abnormal fuel trims.

Pattern 7: P0173 Appears With Other Fuel Trim Codes

P0173 commonly appears alongside:

Multiple fuel trim codes often indicate:

Common Symptoms of P0173

Symptoms vary depending on whether the fuel trim malfunction causes a rich or lean mixture.

Common symptoms include:

Some vehicles initially show only a Check Engine Light.

Check Engine Light

The Check Engine Light is usually the first symptom.

Most manufacturers require:

before storing P0173.

Rough Idle

Fuel trim problems often produce:

Idle quality frequently worsens before other symptoms appear.

Hesitation During Acceleration

Improper fuel trim calculations may cause:

The severity depends on how far fuel trims have moved from normal.

Reduced Fuel Economy

When fuel trims become unstable:

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:

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:

Stop driving immediately if:

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:

  1. Vacuum leak on Bank 2
  2. Intake manifold gasket leak
  3. Dirty MAF sensor
  4. Failed MAF sensor
  5. Low fuel pressure
  6. High fuel pressure
  7. Weak fuel pump
  8. Restricted fuel filter
  9. Faulty fuel pressure regulator
  10. Leaking fuel injectors
  11. Restricted fuel injectors
  12. Exhaust leak before the oxygen sensor
  13. Faulty upstream oxygen sensor
  14. Faulty Air/Fuel Ratio sensor
  15. Stuck-open EVAP purge valve
  16. Faulty MAP sensor
  17. Faulty engine coolant temperature sensor
  18. Dirty throttle body
  19. Restricted air filter
  20. Intake duct leak
  21. PCV system leak
  22. Charging system problems
  23. Low battery voltage
  24. Wiring damage
  25. Poor electrical grounds
  26. Fuel contamination
  27. Incorrect aftermarket intake installation
  28. Engine mechanical problems
  29. PCM software issue
  30. 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:

The next section of this guide covers:

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

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:

Proper diagnosis should identify the root cause before replacing expensive components.

Safety Precautions

Before beginning diagnosis:

Fuel system testing involves pressurized gasoline and should always be performed carefully.

Tools Required

Professional diagnosis may require:

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:

Monitor:

Always save freeze-frame data before clearing codes.

Step 2 – Check for Related Trouble Codes

P0173 commonly appears with:

Related codes often simplify diagnosis.

Code CombinationLikely Diagnosis
P0173 onlyBank 2 fuel trim problem
P0173 + P0174Lean condition affecting Bank 2
P0173 + P0175Rich condition affecting Bank 2
P0173 + P0101Dirty or faulty MAF sensor
P0173 + P0300Fuel 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:

Normal values:

Acceptable:

Problematic:

Severe:

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:

Large differences between the two banks often indicate:

This comparison is one of the quickest ways to narrow the diagnosis.

Step 5 – Inspect the Air Intake System

Inspect:

Look for:

Unmetered air or restricted airflow can significantly alter fuel trim calculations.

Step 6 – Inspect for Vacuum Leaks

Inspect:

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:

Clean the sensor only with MAF sensor cleaner.

Never touch the sensing element.

Step 8 – Monitor MAF Airflow

Observe live airflow.

Typical idle airflow:

Low airflow readings commonly indicate:

Step 9 – Measure Fuel Pressure

Using a fuel pressure gauge:

Compare pressure with manufacturer specifications.

Improper fuel pressure may indicate:

Fuel delivery problems frequently create unstable fuel trim corrections.

Step 10 – Inspect Fuel Injectors

Inspect injectors for:

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:

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:

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:

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:

Freeze-frame data often reveals exactly when the PCM detected abnormal fuel trim behavior.

Step 15 – Inspect Wiring and Connectors

Inspect connectors for:

Look for:

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:

Many P0173 cases begin after unrelated repair work.

Step 18 – Evaluate Engine Mechanical Condition

Perform:

Mechanical problems affecting Bank 2 can force abnormal fuel trim corrections.

Step 19 – Perform a Road Test

Monitor live data during:

Observe:

Fuel trims should remain stable and close to normal throughout the drive cycle.

Step 20 – Confirm the Diagnosis

Before replacing major components, verify:

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:

After repairs:

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:

  1. Turn the ignition off.
  2. Remove the MAF sensor.
  3. Spray the sensing element using MAF sensor cleaner only.
  4. Allow the sensor to dry completely.
  5. Reinstall the sensor.
  6. Clear all DTCs.
  7. 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:

Always verify fuel pressure after repairs before replacing additional components.

Step 24 – Repair Fuel Injector Problems

Inspect injectors for:

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:

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:

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:

Replace the upstream oxygen sensor only after verifying:

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:

Always verify the PCM has the latest available software.

Step 29 – Verify Fuel Trim Operation

After repairs:

Monitor:

Normal fuel trims should generally remain within:

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:

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:

Verifying the Repair

After completing repairs:

  1. Clear all diagnostic trouble codes.
  2. Start the engine.
  3. Allow the engine to reach operating temperature.
  4. Monitor STFT and LTFT for both banks.
  5. Perform idle, cruise, and acceleration testing.
  6. Complete a full OBD-II drive cycle.
  7. Confirm no pending or permanent DTCs return.
  8. 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

RepairTypical 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:

Many P0173 repairs are resolved by correcting airflow or fuel delivery issues rather than replacing major components.

Manufacturer-Specific Notes

Ford

Inspect:

Ford engines commonly develop intake leaks that affect one cylinder bank.

General Motors (Chevrolet/GMC)

Verify:

Fuel trim malfunctions are commonly traced to airflow measurement or injector performance.

Toyota/Lexus

Inspect:

Toyota engines often respond well to MAF cleaning and intake leak repairs.

Honda/Acura

Inspect:

Vacuum leaks and injector restrictions frequently cause abnormal fuel trims.

Nissan/Infiniti

Verify:

Cracked intake tubing and contaminated MAF sensors are common causes.

BMW/Mercedes-Benz

Inspect:

European engines frequently require smoke testing to locate small intake leaks.

Hyundai/Kia

Inspect:

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:

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

Related guides include:

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