Error Code P0156 is defined as O2 Sensor Circuit Malfunction Bank 2 Sensor 2. This refers to the rear catalyst monitoring O2 sensor which evaluates the emissions-reducing capability of the catalytic converter. It is set when the rear O2 sensor signal isn’t showing enough activity to determine fuel control and closed loop fuel management.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II, or vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repair of course, vary from one make/model to another.


The O2 (oxygen) sensor plays an important role for the engine to run properly; it sends information to the PCM of the oxygen content/level of the exhaust. The PCM then uses this information to regulate the fuel in the engine and keep a good air fuel ratio as it regulates the amount of fuel released to the engine. If the O2 sensor is faulty, then it can result to various problems in the engine.

Error Code P0156 describes the problem in Bank 2 Sensor 2 of the O2 sensor, also known as the downstream (post-cat) O2 sensor on Bank 2. It’s composed of four wire sensor, with the PCM providing reference/signal voltage of around 0.5V to the sensor, as well as ground.

The O2 sensor also comes with a heater, which allows the sensor to heat up faster, allowing it to achieve closed loop faster than what it usually takes for the exhaust to warm up the sensor to the right operating temperature. Based on the oxygen level in the exhaust, the O2 sensor will vary the supplied reference voltage. Variation ranges from 0.1 to 0.9, with the former indicating lean, and the latter indicating a rich exhaust.

When the Bank 2, Sensor 2 of the Oxygen sensor is stuck for too long, or is not active at all, then it will trigger the Check Engine light and register the Error Code P0156.

Common Symptoms

Often times problems on the Bank 2 Sensor 2 of the O2 sensor present few symptoms, since they’re only inputs to the PCM to monitor the quality of the catalytic converter and don’t directly control the fuel. As a matter of fact, some cases may not show noticeable symptoms, aside of course, from the illumination of the Check Engine light. Common symptoms however, include:

  • Excessive fuel consumption
  • Excessive smoke coming out of the tailpipe

Possible Causes

This error code can be caused by a lot of factors and problems, such as:

  • Air leaks in the intake
  • Gas leaks in the exhaust
  • Broken or exposed wires
  • Faulty fuel injectors
  • Shorted voltage circuits or wires
  • Engine vacuum leaks
  • Exhaust holes near the O2 sensor
  • O2 sensor wires rubbing against the exhaust
  • Bad O2 sensor

How to Check

This code is checked using an OBD-II scanner, which will also record all troubles codes stored by the PCM. You can then live stream the data from the O2 sensor through the OBD-II scanner and check whether the O2 sensor is working properly or not.

From there you will be able to determine whether the sensor is switching fast as it should be from low to high voltage.

Again, through the OBD-II scanner, live stream the data for the fuel injectors and make sure they’re working properly.

Then, check the engine for any holes where air may escape. This could be a good indication of vacuum leaks or worse, broken vacuum lines, so check the exhaust for holes as well.

How to Fix

Repairs for this error code of course, are as easy and as straightforward as its diagnosis, which are:

  • Repair of vacuum leaks
  • Repair broken, shorted or exposed O2 sensor wires
  • Repair exhaust damage as needed
  • Replace faulty fuel injector
  • Replace the O2 sensor Bank 2 sensor 2

Though not really considered as a serious trouble code, P0156 can definitely increase the vehicle’s fuel consumption and can cause it to release excessive air pollutants into the air.