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P0139 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


Diagnostic Trouble Code (DTC) P0139 stands for an error for the O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)


This is a generic Onboard Diagnostic-II code that means that the O2 Sensor 2 Bank 1 is experiencing a voltage drop below .02 volts for at least seven seconds during the deceleration cut off point. 

Basically, your vehicle’s ECM (Engine Control Module) is making the determination that the sensor’s response is too slow. This will start to trigger the error code and you’ll see the dreaded “check engine” light on your vehicle’s dash.


Normal causes for the P0139 Code:

  • There could be a leak in your vehicle’s fuel injection system, which can cause extra fuel to be left in the exhaust system
  • Faulty rear O2 sensor wiring. The wiring could simply be loose or worn.
  • Bad catalytic converter.


When your vehicle has a P0139 Code, you can expect a few common problems that should help you to isolate the problem:

  • You might experience the engine running rough during deceleration.
  • You may hear your engine knocking and pinging.
  • Excessive smoke coming from your vehicle exhaust.
  • Emission test failure is common.
  • The engine might experience stuttering when trying to accelerate.


The easiest way to diagnose a P0139 problem is to verify it with a OBD-II scanning tool. It’s always best to see if the problem can be duplicated. This will save you time, stress and money. 

If you don’t have a scanning tool just head down to your local auto repair center so a technician can check your vehicle out. The technician will plug the scanner into the vehicle’s OBD-II port to verify the error Code. 

After verification of the P0139 Code, the technician can reset the check engine light and even test drive your vehicle. If the error shows again, then the technician will be able to troubleshoot the matter. 

Duplicating the P0139 Code is very important, as you don’t want to suspect other potential problems too early in the investigation. 

Common mistakes

Mistakes can and do happen when trying to diagnose OBD-II Codes. 

For the P0139 Code, you should try to follow these steps:

  • Check to see if the throttle may be stuck open. If it is, you should always try to fix that problem and see if the error code returns.
  • Inspect the exhaust system for visible leaks or cracks. Be certain to check the catalytic converter for damage.
  • Fuel injectors are another common area to check, as they can easily cause a P0139 Code error. Verify that they are not leaking.
  • The engine needs to be operating normally. That is, there can be no leaks in the vacuum system, which does sometimes occur. Be certain that the vacuum system is thoroughly checked.

The optimum fuel/air mix needs to be around 14.7:1 to make sure that there isn’t another problem that’s triggering the P0139 Code.

How serious is this?

A P0139 Code can be considered serious. The most serious thing you’ll notice when you receive the error code would probably be fuel inefficiency if the O2 sensor is bad. 

If part of the problem involves leaky fuel injectors, then the ECM wouldn’t be able to control the fuel cut off. This in turn would result in increasingly terrible gas mileage after a while. 

You will also probably notice a smoky exhaust. Rough engine idling when decelerating is another problem your vehicle can experience. 

In the long run, engine performance problems will increase if the problem isn’t fixed as soon as possible. 

What repairs can fix the code?

repair manuals

  • Cleaning, inspecting and replacing fuel injectors if they are found to be leaking
  • It’s also recommended to inspect your vehicle’s entire fuel system
  • Inspecting and or replacing the catalyst in front of the O2 sensor
  • Replace the O2 sensor for bank 1 sensor 2 if all other systems have no problems

Related codes

P0136 – O2 Sensor Circuit (Bank 1 Sensor 2)

P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

P0140 – O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

P0141 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)


Problems with your O2 Sensor Circuit need to be addressed quickly, Bad gas mileage, smoke coming from your exhaust, and rough idling really needs to be handled as quickly as possible.

Your local auto garage technician should be able to easily isolate a P0139 problem and keep your vehicle running smooth in no time. 

An OBD-II scanner and a competent auto repair technician will be your most important weapons to help diagnose and repair the issue with your O2 sensor. By first isolating the possible causes of the code first instead of just guessing at everything under the sun, you should be able to fix your P0139 Code in no time.   

P0139 Code – What Does It Mean & How To Fix It