P0131: What it means and how to fix it

Learn what a P0131 code means and how to fix it

A P0131 code is defined as: “Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 1).” This sensor is located on the engine bank that houses the #1 cylinder and is located BEFORE the catalytic converter

The PCM sends a reference voltage (usually 5-volts) to the variable resistor in the oxygen sensor and looks for a return voltage. Once the sensor is heated up, the PCM expects to see a varying voltage. If the PCM sees a constantly low voltage, it will set a trouble code P0131.

The most common causes are a vacuum leak that allows too much-unmetered air into the combustion chamber and causes lean misfires, a bad oxygen sensor, or a poor connection either at the PCM or O2 sensor.

Avoid this mistake when dealing with a P0131 code

Many DIYers automatically replace the oxygen sensor when they see this code. The sensor may be bad. But it may also be setting this code because you have a vacuum or exhaust leak or a low fuel pressure problem. To avoid replacing a good sensor, always start your diagnosis by checking for a disconnected vacuum line or a leak in the exhaust system. In addition, check for low fuel pressure that would cause the engine to run lean, resulting in a consistently low reading at the oxygen sensor.

TEST: It is very difficult to test an oxygen sensor without a scope or scan tool.

The most effective way to determine if you have a vacuum leak is to examine the fuel trim reading on a scan tool. If you have a vacuum leak and the sensor detects a continuous lean exhaust condition, the PCM will add fuel to compensate. Fuel trim should be as low as possible but never approaching 25%, which would indicate a vacuum leak.

To learn how to replace an oxygen sensor, click here

© 2012 Rick Muscoplat