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P0139 Code Explained: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) Causes, Symptoms & Fixes

P0139 code illustration showing a slow-responding downstream oxygen sensor on Bank 1 Sensor 2 with a catalytic converter, Check Engine Light, Pro Street TV logo, and www.ProStreetOnline.com.

If your OBD-II scanner displays P0139, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the Bank 1 Sensor 2 oxygen (O2) sensor is responding slower than the manufacturer allows.

The generic OBD-II definition for P0139 is:

O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

Bank 1 Sensor 2 is the downstream oxygen sensor located after the catalytic converter on the engine bank containing cylinder number one.

Unlike Bank 1 Sensor 1, which rapidly adjusts fuel delivery, Bank 1 Sensor 2 primarily monitors:

When the PCM determines that the downstream oxygen sensor changes voltage too slowly, it stores P0139 and usually illuminates the Check Engine Light.

Although an aging oxygen sensor is the most common cause, slow response can also result from:

Just because the sensor is slow doesn’t necessarily mean it’s lazy. Sometimes the wiring, heater circuit, or exhaust system is making it look that way.

What Does the P0139 Code Mean?

The downstream oxygen sensor measures the amount of oxygen remaining in the exhaust gases after they pass through the catalytic converter.

Unlike the upstream sensor, which constantly switches between rich and lean conditions, the downstream sensor normally changes voltage more gradually because the catalytic converter smooths oxygen fluctuations.

With P0139, the PCM has determined that the downstream oxygen sensor does respond, but not quickly enough.

Instead of reacting within the expected time, the sensor lags behind changes occurring in the exhaust stream.

Possible reasons include:

P0139 identifies a slow-response condition, not a complete sensor failure.

Where Is Bank 1 Sensor 2 Located?

Bank 1 Sensor 2 is always located after the catalytic converter.

Depending on vehicle design, it may be installed in the:

On inline engines:

Bank 1 is the only engine bank.

On V-style engines:

Bank 1 is the side containing cylinder number one.

Sensor 2 always identifies the downstream oxygen sensor mounted behind the catalytic converter.

Bank 1 Sensor 1 vs. Bank 1 Sensor 2

These sensors perform completely different functions.

Bank 1 Sensor 1

Located before the catalytic converter.

Responsible for:

Bank 1 Sensor 2

Located after the catalytic converter.

Responsible for:

While the upstream sensor actively manages combustion, the downstream sensor confirms that the catalytic converter is cleaning up the results.

How the Downstream Oxygen Sensor Works

Most downstream oxygen sensors are zirconia narrowband sensors.

The sensing element compares:

The difference generates a voltage signal.

Typical voltage range:

Because exhaust gases have already passed through the catalytic converter, the downstream sensor should change voltage smoothly and gradually rather than rapidly.

What Does “Slow Response” Mean?

A slow-response oxygen sensor still works—but not quickly enough.

The PCM expects the downstream sensor to react within a calibrated amount of time as exhaust conditions change.

Instead, the sensor may:

As oxygen sensors age, their internal ceramic sensing elements become less responsive, making P0139 increasingly common on higher-mileage vehicles.

How the PCM Detects P0139

The PCM monitors Bank 1 Sensor 2 during specific operating conditions.

Typical monitoring requirements include:

The PCM measures how long the downstream sensor takes to respond to changing exhaust conditions.

If the response exceeds the manufacturer’s calibrated limit, P0139 is stored.

Some vehicles require multiple failed drive cycles before illuminating the Check Engine Light.

Why the Downstream Oxygen Sensor Matters

Although Bank 1 Sensor 2 does not directly adjust fuel injector pulse width, it plays an essential role in emissions monitoring.

The PCM uses it to verify:

Without an accurate downstream oxygen sensor, the PCM cannot reliably evaluate converter performance.

Can an Aging Oxygen Sensor Cause P0139?

Absolutely.

As oxygen sensors accumulate mileage, the sensing element gradually becomes less responsive.

Common reasons include:

Many original oxygen sensors begin slowing noticeably after 100,000 to 150,000 miles, although operating conditions greatly affect lifespan.

Can a Catalytic Converter Cause P0139?

Yes.

Although P0139 primarily identifies a slow downstream oxygen sensor, catalytic converter problems may contribute.

Possible converter-related causes include:

Catalytic converter failures more commonly trigger P0420, but the converter should still be evaluated during diagnosis.

P0139 Quick Facts

ItemInformation
DTCP0139
Generic DefinitionO2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
SystemFuel & Emissions
Sensor LocationDownstream (After Catalytic Converter)
SeverityLow to Moderate
Safe to Drive?Usually Yes
Common SymptomsCheck Engine Light, emissions failure, catalyst monitor problems
Common CausesAging oxygen sensor, contamination, wiring damage, heater circuit faults
Typical Repair Cost$100–900+
Related CodesP0136, P0137, P0138, P0140, P0141, P0420

Common Vehicles That Experience P0139

Because P0139 is a generic OBD-II diagnostic trouble code, it may occur on nearly every gasoline-powered vehicle built since 1996.

Common manufacturers include:

Vehicles with higher mileage are particularly susceptible because downstream oxygen sensors naturally become slower with age.

What Does P0139 Really Mean?

The wording of P0139 often leads drivers to assume the downstream oxygen sensor has completely failed.

The code does not automatically mean:

Instead, it means the PCM has determined that Bank 1 Sensor 2 is reacting too slowly to normal exhaust changes.

The problem may involve:

Think of P0139 as the PCM saying:

“The downstream oxygen sensor is still talking to me—it just isn’t keeping up with the conversation anymore.”

Next Section

Part 1B covers:

Common P0139 Scan Tool Patterns

Monitoring live scan-tool data is one of the most effective ways to diagnose P0139.

Unlike the upstream oxygen sensor, Bank 1 Sensor 2 should not switch rapidly. Because it monitors exhaust gases after they pass through the catalytic converter, its voltage changes should be slow, smooth, and stable.

With P0139, the problem is not that the sensor has stopped working—it is that the sensor is responding slower than the PCM expects.

Pattern 1: Slow Voltage Changes

The most common scan-tool pattern is a downstream oxygen sensor that eventually changes voltage but does so much more slowly than normal.

Instead of responding within seconds, the sensor may:

Possible causes include:

This is the classic scan-tool pattern associated with P0139.

Pattern 2: Delayed Rich-to-Lean Response

Momentarily creating a lean condition should cause the downstream oxygen sensor voltage to decrease.

A healthy sensor should:

A slow sensor may:

Delayed voltage transitions are a defining characteristic of P0139.

Pattern 3: Delayed Lean-to-Rich Response

Momentarily enriching the air-fuel mixture should cause Bank 1 Sensor 2 voltage to increase.

A healthy sensor gradually rises toward:

A slow sensor may:

This commonly indicates an aging oxygen sensor.

Pattern 4: Downstream Sensor Lags Behind Upstream Sensor

Monitor:

Normal operation:

With P0139:

The downstream sensor appears to “follow” the upstream sensor but with excessive delay.

Pattern 5: Slow Response After Warm-Up

Some vehicles operate normally immediately after startup.

As the engine reaches operating temperature:

Possible causes include:

Pattern 6: Catalyst Monitor Incomplete

Many scan tools display:

Because the downstream sensor cannot respond quickly enough, the PCM may suspend catalyst monitoring.

Common Symptoms of P0139

Most vehicles continue to drive normally because the downstream oxygen sensor does not directly control fuel delivery.

Common symptoms include:

Many drivers notice only the illuminated Check Engine Light.

Check Engine Light

The Check Engine Light is almost always the first symptom.

Most manufacturers require:

before storing P0139.

Failed Emissions Inspection

P0139 frequently prevents successful emissions testing because:

A perfectly smooth-running engine can still fail an emissions inspection because one sensor is taking its time.

Catalyst Monitor Problems

Bank 1 Sensor 2 is critical for catalyst monitoring.

When response time becomes excessive:

Slight Reduction in Fuel Economy

Although the downstream oxygen sensor does not directly manage fuel delivery, some vehicles use its information for adaptive fuel strategies.

Drivers may notice:

Fuel economy changes are generally modest compared to upstream oxygen sensor faults.

Mild Hesitation

Some vehicles may exhibit:

These symptoms usually indicate another engine issue contributing to the slow sensor response.

Is P0139 Serious?

P0139 is generally considered a low-to-moderate severity diagnostic trouble code.

Most vehicles remain fully drivable.

However, delaying repairs may result in:

Prompt diagnosis is recommended to maintain proper emissions system performance.

Can You Drive With P0139?

Usually yes.

Driving is generally safe if:

Repairs should be scheduled soon if:

P0139 vs Related Trouble Codes

Understanding related downstream oxygen sensor codes helps narrow the diagnosis.

P0136

O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)

General circuit fault.

P0137

O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

Voltage remains too low.

P0138

O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

Voltage remains too high.

P0139

O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

Sensor reacts too slowly to exhaust changes.

P0140

O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

No usable sensor activity.

P0141

O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

Heater circuit failure.

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

Catalytic converter efficiency problem.

30 Common Causes of P0139

The most common causes include:

  1. Aging Bank 1 Sensor 2 oxygen sensor
  2. Sensor contamination
  3. Carbon buildup
  4. Oil contamination
  5. Coolant contamination
  6. Silicone contamination
  7. Weak heater circuit
  8. Failed heater element
  9. Blown heater fuse
  10. Damaged heater wiring
  11. Corroded connector terminals
  12. Loose electrical connector
  13. High circuit resistance
  14. Heat-damaged wiring
  15. Exhaust leaks
  16. Catalytic converter deterioration
  17. Restricted catalytic converter
  18. Poor sensor ground
  19. Damaged wiring harness
  20. Water intrusion into connector
  21. Road debris damage
  22. Incorrect aftermarket oxygen sensor
  23. Previous poor wiring repairs
  24. Rich engine operation
  25. Lean engine operation
  26. Faulty Mass Air Flow (MAF) sensor
  27. Fuel delivery problems
  28. PCM calibration issue
  29. Damaged PCM connector
  30. PCM failure (rare)

Final Thoughts

The P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) code indicates the PCM has determined that the downstream oxygen sensor is reacting too slowly to changes in exhaust oxygen levels.

Although an aging oxygen sensor is the most common cause, contamination, heater circuit faults, damaged wiring, connector corrosion, exhaust leaks, catalytic converter problems, and electrical faults can all produce the same slow-response condition.

Proper diagnosis should determine whether the problem originates in:

The next section of this guide covers:

Following a structured diagnostic process prevents unnecessary parts replacement while restoring proper catalyst monitoring and emissions system performance.

Related Pro Street Diagnostic Guides

Continue to Part 2 for complete diagnostic procedures, repair methods, repair costs, manufacturer-specific information, and frequently asked questions.

How to Diagnose the P0139 Code

Diagnosing P0139 requires determining why the Bank 1 Sensor 2 oxygen sensor is responding more slowly than the PCM expects.

Unlike the upstream oxygen sensor, which directly controls fuel delivery, the downstream oxygen sensor is primarily responsible for monitoring catalytic converter efficiency and verifying proper emissions system operation.

Although an aging downstream oxygen sensor is the most common cause of P0139, slow response may also result from:

Replacing the oxygen sensor without confirming the actual cause may solve the problem—or simply become an expensive guessing game.

Safety Precautions

Before beginning diagnosis:

The downstream oxygen sensor operates in an environment that routinely exceeds 900°F (480°C).

Tools Required

Professional diagnosis may require:

An oscilloscope provides the most accurate method of measuring oxygen sensor response time.

Step 1 – Verify the Code

Connect a professional scan tool.

Record:

Monitor:

Do not clear any codes until freeze-frame information has been recorded.

Step 2 – Check for Related Trouble Codes

P0139 frequently appears with:

Multiple DTCs often point toward the root cause.

Code CombinationLikely Diagnosis
P0139 onlyAging downstream oxygen sensor
P0139 + P0141Heater circuit fault
P0139 + P0420Catalytic converter efficiency problem
P0139 + P0136General downstream sensor circuit fault
P0139 + P0171/P0172Fuel mixture affecting sensor response

Step 3 – Verify Closed-Loop Operation

Allow the engine to reach normal operating temperature.

Monitor:

The PCM should transition from:

Open Loop

to

Closed Loop

If closed-loop operation never begins, inspect:

Downstream oxygen sensor response testing should always be performed during closed-loop operation.

Step 4 – Monitor Bank 1 Sensor 2 Live Data

Observe downstream oxygen sensor voltage.

A healthy Bank 1 Sensor 2 should:

Watch for:

These are classic indicators of P0139.

Step 5 – Compare Upstream and Downstream Sensors

Monitor:

Normal operation:

With P0139:

This comparison helps isolate the fault to the downstream sensor rather than the fuel control system.

Step 6 – Perform Rich and Lean Response Tests

Momentarily create:

A healthy downstream sensor should gradually respond in both directions.

Watch for:

Slow bidirectional response strongly suggests sensor deterioration or heater problems.

Step 7 – Analyze Fuel Trim Data

Monitor:

Abnormal fuel trims may indicate:

Fuel trim analysis helps determine whether slow sensor response is caused by engine operation or by the sensor itself.

Step 8 – Inspect the Oxygen Sensor

Inspect Bank 1 Sensor 2 for:

Contaminants can significantly slow sensor response without causing a complete failure.

Step 9 – Inspect the Electrical Connector

Disconnect the connector.

Inspect for:

Even slight connector resistance can noticeably slow oxygen sensor signal transmission.

Step 10 – Test the Signal Circuit

Using a digital multimeter:

Back-probe the signal wire.

Inspect for:

Excessive circuit resistance can delay sensor voltage changes enough to trigger P0139.

Step 11 – Verify Sensor Ground

Perform voltage-drop testing.

Inspect:

Poor grounds often produce unstable or sluggish oxygen sensor signals.

Step 12 – Test the Heater Circuit

The downstream oxygen sensor contains an internal heater.

Inspect:

A weak heater slows sensor warm-up and reduces response speed during normal operation.

Step 13 – Inspect the Exhaust System

Inspect the exhaust system around Bank 1 Sensor 2.

Check for:

Exhaust leaks introduce fresh oxygen that can alter sensor response characteristics.

Step 14 – Evaluate Catalytic Converter Performance

Compare upstream and downstream oxygen sensor activity.

A healthy catalytic converter should:

If converter efficiency is reduced, the downstream sensor may respond abnormally slowly.

Step 15 – Inspect the Wiring Harness

Inspect the complete harness between:

Look for:

Heat-related wiring damage remains one of the most common causes of downstream oxygen sensor circuit issues.

Step 16 – Review Technical Service Bulletins

Before replacing expensive components:

Review manufacturer Technical Service Bulletins.

Look for updates involving:

Some manufacturers have released software updates that improve oxygen sensor diagnostics.

Step 17 – Compare Bank 2 Sensor 2 (If Equipped)

On V-engine applications:

Compare:

If Bank 2 responds normally while Bank 1 is noticeably slower, the fault is likely isolated to Bank 1 Sensor 2 or its circuit.

Step 18 – Review Freeze-Frame Data

Compare current live data with recorded freeze-frame information.

Review:

Freeze-frame data often identifies the operating conditions that caused the PCM to detect slow sensor response.

Step 19 – Confirm Sensor Switching Time

Using a graphing scan tool or oscilloscope:

Measure how long the downstream oxygen sensor takes to:

Response times that exceed manufacturer specifications confirm the slow-response condition.

Step 20 – Confirm the Diagnosis

Before replacing any components, verify:

A thorough diagnosis prevents unnecessary replacement of oxygen sensors, catalytic converters, and PCM components.

Next Section

Part 2B covers:

Step 21 – Replace the Downstream Oxygen Sensor (If Required)

Replace Bank 1 Sensor 2 only after confirming that the downstream oxygen sensor has become sluggish or that testing verifies an internal sensor failure.

General replacement procedure:

  1. Allow the exhaust system to cool completely.
  2. Disconnect the negative battery cable if recommended by the manufacturer.
  3. Disconnect the downstream oxygen sensor electrical connector.
  4. Remove the sensor using an oxygen sensor socket.
  5. Inspect the sensor threads and mounting bung.
  6. Install a quality OEM or OEM-equivalent replacement sensor.
  7. Tighten the sensor to the manufacturer’s specified torque.
  8. Reconnect the electrical connector.
  9. Clear all stored diagnostic trouble codes.
  10. Verify proper sensor response using a professional scan tool.

OEM oxygen sensors typically provide the fastest and most accurate response times for catalyst monitoring.

Step 22 – Repair the Signal Circuit

High electrical resistance can slow oxygen sensor response enough to trigger P0139.

Inspect the signal circuit for:

Repair using:

Even minor wiring damage can significantly delay downstream oxygen sensor response.

Step 23 – Repair Connector Problems

Disconnect the Bank 1 Sensor 2 connector.

Inspect for:

Poor electrical connections increase circuit resistance and commonly contribute to slow-response codes.

Replace damaged connectors rather than attempting temporary repairs.

Step 24 – Repair the Heater Circuit

A weak heater circuit is one of the most overlooked causes of P0139.

Inspect:

A heater that cannot maintain proper sensor temperature slows voltage response and may trigger both P0139 and P0141.

Step 25 – Repair Exhaust Leaks

Inspect the exhaust system around Bank 1 Sensor 2.

Repair:

Fresh outside air entering the exhaust stream can delay downstream oxygen sensor response and interfere with catalyst monitoring.

Step 26 – Clean Up Engine Contamination

Slow oxygen sensors are often contaminated rather than electrically damaged.

Inspect for evidence of:

Correct the source of contamination before installing a replacement sensor to avoid repeated failures.

Step 27 – Repair Ground Circuit Problems

Poor electrical grounds can slow sensor response.

Perform voltage-drop testing.

Inspect:

Repair loose, corroded, or damaged ground connections before replacing expensive components.

Step 28 – Inspect the Catalytic Converter

Although P0139 is primarily a downstream oxygen sensor code, catalytic converter problems can contribute to slow sensor response.

Inspect for:

If P0420 is also present, perform catalyst efficiency testing before replacing the converter.

Step 29 – Update PCM Software

Before replacing major components, review manufacturer Technical Service Bulletins (TSBs).

Manufacturers occasionally release software updates affecting:

Always verify the PCM has the latest available calibration.

Step 30 – Consider PCM Failure

PCM failure is extremely rare.

Only suspect the PCM after confirming:

PCM replacement should always be considered the final diagnostic step.

How to Fix P0139

The correct repair depends entirely on 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. Verify closed-loop operation.
  5. Monitor Bank 1 Sensor 2 response time.
  6. Compare upstream and downstream sensor activity.
  7. Perform multiple rich-to-lean and lean-to-rich response tests.
  8. Complete a full road test.
  9. Verify all readiness monitors complete successfully.
  10. Confirm no pending or permanent DTCs remain.

A successful repair restores proper downstream oxygen sensor response speed and accurate catalyst monitoring.

Typical Repair Costs

RepairTypical Cost
Professional diagnosis$100–250
Bank 1 Sensor 2 replacement$175–450
Wiring repair$150–500
Connector replacement$100–350
Heater circuit repair$150–500
Exhaust leak repair$150–800
Ground circuit repair$100–300
PCM software update$100–300
Catalytic converter replacement$900–2,800+
PCM replacement$1,000–3,000+

Repair costs vary depending on vehicle make, labor rates, and parts availability.

Common Diagnostic Mistakes

Avoid these common mistakes:

Many P0139 repairs involve correcting heater, wiring, or contamination issues rather than replacing the sensor itself.

Manufacturer-Specific Notes

General Motors

Inspect:

Heat-damaged wiring is common on older GM platforms.

Ford

Verify:

EcoBoost engines should also be inspected for exhaust leaks around turbocharger components.

Toyota / Lexus

Inspect:

Toyota applications generally achieve the best results using OEM oxygen sensors.

Honda / Acura

Inspect:

Some aftermarket sensors may respond slower than OEM units and trigger P0139.

Nissan / Infiniti

Compare:

High-mileage Nissan vehicles commonly develop slower downstream oxygen sensors due to age.

Hyundai / Kia

Inspect:

Road salt and moisture frequently accelerate connector corrosion.

Volkswagen / Audi

Verify:

Turbocharged engines require careful inspection of the catalytic converter and post-catalyst sensor.

BMW / MINI

Use factory diagnostic software to monitor:

Manufacturer-specific test routines provide the most accurate diagnosis.

Chrysler / Dodge / Jeep / Ram

Inspect:

Heat and vibration commonly damage rear oxygen sensor wiring on high-mileage vehicles.

Frequently Asked Questions

What usually fixes P0139?

The most common repairs include replacing an aging downstream oxygen sensor, repairing damaged wiring, fixing heater circuit faults, replacing corroded connectors, repairing exhaust leaks, or correcting contamination that slows sensor response.

Can I drive with P0139?

Yes. Most vehicles remain drivable, but emissions may increase, catalyst monitoring may stop functioning correctly, and the vehicle may fail an emissions inspection.

Does P0139 always mean the oxygen sensor is bad?

No. Wiring resistance, heater circuit failures, exhaust leaks, connector corrosion, contamination, and catalytic converter problems can all slow oxygen sensor response without the sensor itself being defective.

Can an exhaust leak cause P0139?

Yes. Exhaust leaks near the downstream oxygen sensor can delay voltage changes and interfere with catalyst monitoring, causing the PCM to detect a slow-response condition.

Can contamination cause P0139?

Absolutely. Oil, coolant, silicone, and heavy carbon deposits can coat the sensing element and dramatically reduce oxygen sensor response speed.

Can a catalytic converter cause P0139?

Sometimes. Although P0139 primarily identifies a slow downstream oxygen sensor, catalytic converter deterioration can affect downstream oxygen storage characteristics and contribute to slow response. Converter failures more commonly trigger P0420.

Final Thoughts

The P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) code indicates the PCM has determined that the downstream oxygen sensor is responding too slowly to changing exhaust conditions. While sensor aging is the most common cause, heater circuit faults, contamination, wiring resistance, connector corrosion, exhaust leaks, and catalytic converter issues can all produce the same condition.

A complete diagnosis should always verify:

Following a systematic diagnostic process restores accurate catalyst monitoring, prevents unnecessary parts replacement, improves emissions compliance, and ensures the vehicle’s OBD-II readiness monitors complete successfully.

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