Diagnosing And Fixing P0139 Honda Accord – Oxygen Sensor Slow Response Code

The P0139 Honda Accord code indicates a slow response from the rear oxygen sensor. This can be caused by a faulty oxygen sensor, wiring issues, or exhaust leaks. Replacing the sensor or fixing exhaust leaks will typically resolve the problem.

The P0139 Honda Accord oxygen sensor diagnostic trouble code (DTC) is one you don’t want to ignore. It indicates that there is an issue with the Bank 1 Sensor 2 oxygen sensor’s response rate. This sensor measures the oxygen content of the exhaust stream. It helps the engine control module (ECM) adjust the air/fuel ratio for optimal performance and emissions. When it malfunctions, engine performance suffers.

What Does The P0139 Code Mean?

The P0139 code means that the Bank 1 Sensor 2 oxygen sensor is taking too long to respond to changes in exhaust oxygen content. This sensor is located after the catalytic converter.

There are several failure modes that can cause slow response:

  1. The oxygen sensor is contaminated or losing sensitivity
  2. The sensor is faulty and needs to be replaced
  3. There is increased resistance in the sensor circuit wiring
  4. The sensor connector or terminals are damaged or corroded

The slow switching response time between rich (low oxygen) and lean (excess oxygen) conditions is detected by the ECM. If it exceeds the ECM thresholds, the P0139 code is set.

Bank 1 refers to the side of the engine where cylinder #1 is located in the firing order on Honda Accord models.

Sensor 2 is the engine’s second oxygen sensor – the downstream sensor located after the catalytic converter.

What Are the Symptoms of A P0139 Honda Accord?

The P0139 itself will illuminate the check engine light. However, an oxygen sensor response problem can cause other symptoms:

  1. Reduced fuel economy
  2. Misfiring, hesitation, rough idle
  3. Failed emissions inspections (due to excess emissions)
  4. Exhaust smells “rotten egg” (caused by high hydrocarbon emissions)
What Are the Symptoms of A P0139 Honda Accord
What Are the Symptoms of A P0139 Honda Accord

Because the oxygen sensor issue is likely intermittent and resets after restarting the engine, the service engine light may illuminate without any obvious symptoms. However, the underlying issue remains.

Failure to diagnose and repair a P0139 Honda Accord code as soon as possible can result in catalytic converter damage due to oxygen sensor issues.

What Causes The P0139 Code On Honda Accord?

Here are the most common causes of the P0139 Honda Accord trouble code:

  1. Failed or Failing Oxygen Sensor
  2. Damaged Sensor Wiring
  3. Exhaust Leaks

Failed or Failing Oxygen Sensor

The oxygen sensor itself is the most likely root cause of P0139. The downstream oxygen sensors have a tougher operating environment than the upstream sensors. They are exposed to higher temperatures and contaminants in the exhaust stream:

  1. The high heat can cause internal degradation and component failure
  2. Contaminants from fuel, oil, and engine wear can coat and “blind” the sensor over time

A failing sensor will react slowly to changes in oxygen levels. The replacement restores the fast switching response. Plan on replacing oxygen sensors every 100,000 miles. Only use genuine Honda oxygen sensors. Aftermarket components frequently fail prematurely..

Damaged Sensor Wiring

The oxygen sensor connectors and wiring are exposed to high underhood temperatures. The wires can rub against hot engine components and become damaged over time. This causes resistance and voltage drops in the circuit:

  1. Check for corroded or damaged wiring and connectors
  2. Wiring insulation may be cracked or melted due to heat damage
  3. Fix any pinched, cut, or scraped wires

Carefully inspect the connectors. Make sure the terminals are tight and free of corrosion.

Exhaust Leaks

The exhaust manifold, catalytic converter, and downstream piping must be leak-free. Any leaks will allow false air to mix with exhaust gases:

  1. This skews the oxygen sensor readings
  2. It will detect less oxygen in the diluted exhaust stream
  3. The ECM receives inaccurate data for fuel trim adjustments

Inspect the exhaust system carefully. Look for cracks, pinholes, gasket leaks, or broken welds. Repair any leaks found to prevent false readings.

How To Diagnose P0139 Honda Accord

To locate the root cause of the P0139 Honda Accord code, a systematic approach is required. Begin by connecting a scan tool and looking for any other stored diagnostic trouble codes that could be causing the problem. Drive the vehicle and note any noticeable symptoms such as a rough idle, a lack of power, an exhaust odor, and so on.

Next, look for any damage, corrosion, or loose connections in the oxygen sensor wiring harness. Wiggle the wiring while watching the voltage signals to see if there are any dropouts or intermittent issues. Make sure the sensor connector is completely seated and locked into the receptacle.

Check the sensor heater circuit and signal voltage circuits with a digital multimeter for any voltage or ground shorts. For specifications, consult the factory wiring diagram. Check for continuity from the sensor connector to the PCM via the wiring harness.

If wiring faults have been ruled out, use a digital multimeter to test sensor resistance. Check the sensor internal resistance readings at operating temperature against the factory specifications. Sensor resistance that is out of range or unstable indicates degradation and the need for replacement.

Examine the entire exhaust system for any leaks that could allow false air into the post-cat oxygen sensor. Even minor leaks can have an impact on the sensor’s switching response. Check all clamps, hangers, and flange hardware for proper torque. Any leaking components should be repaired or replaced.

How To Fix P0139 Honda Accord

Once the root cause of the slow oxygen sensor response has been identified, perform the necessary repairs to clear the P0139 code. Replace the post-cat O2 sensor with a new Honda genuine oxygen sensor if it is found to be contaminated or unstable. On the sensor body, use a sensor-specific wrench and the appropriate torque.

Repair or replace any damaged wiring that is discovered. Keep wires from kinking and keep them away from hot or moving components. Wrap any exposed areas in heat shielding wrap. Check that all of the electrical connections are clean and tight.

How To Fix A P0139 Honda Accord
How To Fix A P0139 Honda Accord

To address exhaust leaks, replace any worn or misaligned gaskets, rings, or seals. Replace any rusted or cracked exhaust sections. Confirm that the slip joints are properly fitted. Apply anti-seize compound to the oxygen sensor threads and tighten to the manufacturer’s torque specifications. Clear codes and perform road tests to ensure that the oxygen sensor is operating normally.

What Is The Code P0139 On A 2006 Honda Accord?

The P0139 code on a 2006 Honda Accord indicates a problem with the Bank 1, Sensor 2 oxygen sensor (O2 sensor). This downstream O2 sensor is located after the catalytic converter. P0139 indicates a slow response from this rear O2 sensor during voltage switching. 

The engine control module (ECM) detects sluggish rich/lean transitions from the sensor and sets the diagnostic trouble code (DTC) if they exceed preset limits. Delays in O2 sensor response can be caused by a faulty sensor, damaged wiring, or exhaust leaks.

What Are The Symptoms Of the P0139 Code?

Typical symptoms of P0139 in a 2006 Accord are check engine light illumination, poor fuel economy, rough idle, rotten egg smell from the exhaust, failed emissions testing, and engine hesitation or misfiring. 

However, the code may also be present with no noticeable symptoms other than the service engine soon light coming on. An oxygen sensor problem allows excess emissions that smell bad and cause driveability issues.

What Is OBD II P0139 Honda?

OBD II P0139 Honda is a diagnostic trouble code that indicates a slow response of the Bank 1, Sensor 2 oxygen sensor on Honda vehicles. It is a generic powertrain code, which means it applies to all Honda models equipped with OBD II after 1996, including the Accord, Civic, CR-V, Odyssey, Pilot, and others. The P0139 DTC indicates abnormally slow voltage switching of the downstream oxygen sensor, which interferes with optimal fuel control.

What Causes A Slow Response O2 Sensor?

A degraded or contaminated sensor, wiring faults such as corroded connectors or frayed wires, and exhaust leaks prior to the sensor are the most common causes of a slow response O2 sensor that triggers P0139 on a Honda. 

Over time, fuel residue, oil ash, and blowby gasses can accumulate on the sensor and slow its reaction rate. Damaged wiring raises circuit resistance, causing sensor signals to slow. Exhaust leaks distort oxygen content measurements.

The Bottom Line

The P0139 Honda Accord trouble code indicates the rear oxygen sensor is responding slowly due to sensor wear, wiring faults, or exhaust leaks. Following structured diagnostic and repair procedures restores rapid O2 sensor switching. This maintains optimal fuel control, engine efficiency, and low emissions.

Proper oxygen sensor operation is crucial for engine performance and longevity. Diagnose P0139 promptly and make necessary repairs. With a sound O2 sensor system, your Honda Accord will run smoothly for years to come.

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Ammar Masoud

I have had a long and fulfilling career in the automotive industry, primarily with Honda and Acura. With 15 years of experience as a Honda service technician, I became highly skilled in repair and maintenance, gaining a deep understanding of these vehicles. After many years in the automotive field, I decided to embark on a second career in industrial manufacturing. It was a significant change, but I found that the skills I had honed in the automotive industry were incredibly valuable in my new role. In my current position in industrial manufacturing, the demand for quality workmanship and meticulous attention to detail is paramount. Fortunately, these are traits that I have cultivated throughout my years in the automotive industry. I take pride in applying these skills to meet the high standards expected in the manufacturing sector.