Error Code P0139 is described as Oxygen Sensor Circuit Slow Response Bank 1, Sensor 2. Meaning, the O2 Sensor Bank 1, Sensor 2 has failed to deliver a voltage drop of 0.2V for 7 seconds at deceleration fuel cut off. It tells the PCM (powertrain control module, also known as ECM or engine control module) that the sensor is responding too slowly.

This code is a generic error code, meaning it applies to all vehicles made since 1996 up to present. Specifications in definition and troubleshooting vary from one make and model to another.


The PCM cuts the fuel to the engine upon deceleration, and O2 sensors should respond accordingly by a voltage output lower than 0.2V, showing exhaust stream as high in O2 content. If the O2 sensor for Bank 1 Sensor 2 doesn’t respond to the fuel cut-off within 7 seconds (or more, depending on the make and model of the vehicle), then it triggers the error code.

This involves the O2 sensor located at the rear side of the driver. It indicates that the O2 sensor signal or the PCM is not adjusting the engine air fuel ratio the way it’s expected to do so, or doesn’t adjust it as often as expected once the engine is warmed up or under normal use condition. In other words, when the up-down “cross-counting” voltage activity of the O2 sensor is too slow, it triggers the error code P0139.

Common Symptoms

Error Code P0139 is quite tricky, as aside from the activation of the Check Engine light, it doesn’t cause drivability problems. In some cases however, the engine may ping and may product excessive levels of NOx. Also, the engine may fail emission tests due to excessive smoke from the exhaust. The vehicle may also run rough on deceleration due to excessive fuel getting on the engine.

Possible Causes

This code is usually caused by excessive fuel in exhaust stream caused by possible leak in the fuel injection system. Other possible causes include:

  • Faulty O2 sensor
  • Faulty rear O2 sensor wiring
  • Exhaust leak
  • Defective Catalytic Converter

How to Check

As with other error codes, diagnosis for P0139 starts by double-checking the code from the scan tool. A qualified technician will start by clearing the code and take the vehicle for a test drive while duplicating the conditions when the code was set, such as load, speed, RPM etc, and monitor the scan tool if the error code comes back. He will also check the O2 sensor data to see whether the voltage drops below 0.2V at deceleration.

Other factors that should be checked include:

  • The engine fuel pressure for leaks in the fuel injector
  • The O2 sensor to verify if it’s contaminated with coolant or oil outside
  • Exhaust system’s integrity and any possible problem with the catalyst

Note that manufacturers may have different specifications for proper diagnosis of this code.

How to Fix

If the code comes back after the test drive, then you will likely end up replacing damaged or problematic components that trigger the code. Here are some solutions:

  • Check the exhaust leaks and fix problems
  • Search for broken, frayed or shorted wires and repair accordingly
  • Look for deteriorating or contaminated O2 sensor and replace when necessary
  • Check O2 sensor’s frequency and amplitude (advanced)
  • Check for possible inlet air leaks
  • Check MAF sensor for proper operation
  • Replace catalyst fronting the sensor
  • Test the fuel system and then replace any leaking injector

Though Error Code P0139 is not that bad compared to most error codes, pumping fuel upon deceleration is an expensive inconvenience as it can definitely increase the vehicle’s fuel consumption. It may cause the engine to stall when coming to a stop if there is an excessive amount of fuel leaking into the cylinders.