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Oxygen sensors switch between high and low voltage signals that correspond to the oxygen levels in the exhaust system. Although the signal from the front sensor (sensor 1) changes far more rapidly than that of the rear (sensor 2), both change over time. 

When the rear oxygen sensor mounted on engine bank 1 doesn’t transition between voltage states fast enough, the engine computer logs a P0139 code.

What Does the P0139 Code Mean?

Diagnostic trouble code (DTC) P0139 stands for “O2 Sensor Circuit Slow Response (Bank 1, sensor 2).” It indicates that the powertrain control module (PCM) has detected a potential issue with the signal from the rear oxygen sensor on bank 1. If the O2 sensor output is sluggish, the ECM/PCM knows it can’t be trusted to provide real time accurate information.

closeup of an oxygen sensor
Diagnostic trouble code P0139 indicates that the powertrain control module (PCM) has detected a potential issue with the signal from the rear oxygen sensor on bank 1. 

The rear sensor is located behind the catalytic converter, and bank 1 refers to the side of the engine that houses the #1 cylinder.  

In GM automotive terminology, “cross-counts” refers to the oxygen sensor’s voltage activity as the device alternates between a state of high voltage representing a rich mixture and the opposite low voltage state that indicates a lean mixture. A sensor must produce a minimum amount of cross-counting activity to prove that it works properly.

The downstream or rear oxygen sensor doesn’t fluctuate back and forth like the front sensor. But the rear sensor’s signal should vary somewhat over time. If the rear sensor’s signal is slow to change or does not change at all, the PCM may log code P0139. In other words, the ECM/PCM knows how rapidly the downstream O2 sensor should be switching based on the faster switching upstream sensor. It’s a rationality check algorithm.

Note: The definition of code P0139 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P0139 Code?

Several issues can slow down the activity of the bank 1 sensor 2 unit. The possible causes of the P0139 code include:

  • Faulty bank 1 sensor 2 oxygen sensor
  • Issues with the rear oxygen sensor wiring or connections
  • Leaks in the exhaust system
  • A lean or rich running condition 
  • An issue with the PCM, such as software in need of an update 
female mechanic checking exhaust system
Exhaust system leaks can cause P0139 code.

What are the Common Symptoms of Code P0139?

Even before you plug a scan tool to your car’s OBD port, you can get a heads-up that your vehicle suffered an issue that led to a P0139 code. Keep an eye out for these warning signs:

In most cases, your vehicle will run and handle normally since a faulty bank 1 oxygen sensor 2 unit won’t adversely affect engine performance. You should still bring your ride in for diagnosis and repair as soon as possible since you may end up breaking the law if you drive your car with its high emission levels.

How to Diagnose the P0139 Code

Once their OBD-II scanner shows the P0453 code or another diagnostic trouble code, most people seek out professional help in repairing their vehicle. Those who possess sufficient confidence and experience in DIY car repair can diagnose their vehicle and restore the proper cross-counting voltage activity of the bank 1 sensor 2 oxygen sensor.

Watching the following videos about the P0139 code can help you get an idea of what the diagnosis process might involve:

How to fix P0139 on 2000 Nissan Maxima:

Diagnosing P0138 and P0139:

Using a Scan Tool Record Utility

If you have DIY experience, you may also try the diagnostic method below:

Code P0139 that points to a problem with the downstream O2 sensor, which is lazier than the upstreamm sensor on a system where the catalyst is storing oxygen correctly.

If you’re dealing with any kind of concern you believe might be related to the upstream O2 sensor performance and you believe the sensor might need replacing, you can watch the sensor on the scan tool data screen with the throttle at about 1200 rpm and you should see about three switches from rich to lean per second, with the voltage range being at least from 0.2 to 0.8 volts or slightly greater. The downstream sensor will switch as well, only not as much and not as quickly.

The O2 sensor will switch more slowly at idle much of the time, so you’ll need to monitor the sensor with the engine speed elevated and even record it while driving with the engine at highway cruise. Again, either record the pattern while driving using the scan tool record utility or have an assistant driving. DO NOT watch the scan tool screen while driving the vehicle.

The downstream sensor also needs to switch quickly whenever it needs to, and the ECM watches both sensors very carefully to determine if either sensor is responding to fuel trim adjustments at a less-than-optimum rate.

How to Fix the P0139 Code

After diagnosis, the next step of the P0139 code repair process is identifying the right way to fix the issue. Consult online auto repair resources and guides. You can also invest in an ALLDATA single-vehicle subscription for comprehensive factory repair information.  

Because this O2 sensor code is supported by various makes and models, you need to make sure the repair process you’re implementing is appropriate for your vehicle. Check your owner’s manual before doing anything. When in doubt, don’t hesitate to reach out to your mechanic.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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