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Oxygen sensors measure the amount of oxygen in the exhaust stream. Your car’s primary computer, which is often referred to as the powertrain control module (PCM), uses the signal from the upstream oxygen sensor (sensor 1) when calculating fuel control.

When the #1 sensor on engine Bank 1 produces insufficient voltage, the PCM may record the diagnostic trouble code P0131.

What Does the P0131 Code Mean?

Diagnostic trouble code (DTC) P0131 stands for “O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1).” It indicates that the voltage of the affected oxygen sensor stayed below a certain threshold or the air-fuel ratio sensor stayed in a lean-biased mode for too long.

Note: Although code P0131 is a generic code specified by the Society of Automotive Engineers (SAE), the code’s definition may vary depending on the vehicle manufacturer.

Electric transmission control module designed for automatic transmission inside look
To strike a balance between generating enough power for the vehicle and producing air pollution, the PCM checks the oxygen content in the exhaust gases heading towards the catalytic converter.

The engine must strike a balance between generating enough power for the vehicle and producing air pollution. To achieve this, the PCM checks the oxygen content in the exhaust gases heading towards the catalytic converter.

A low voltage signal from the oxygen sensor indicates low oxygen content in the exhaust—a sign that the engine runs rich. The PCM responds by reducing the fuel going to the engine.

Conversely, a high voltage signal shows high oxygen content and warns that the engine runs lean, leading to the PCM increasing fuel to compensate for the excess air.

To learn more about how your vehicle’s computer detects issues with oxygen sensors, you can read our technical explanation here.

What’s the Difference Between Oxygen Sensors and Air-Fuel Ratio Sensors?

Newer vehicles may come with air-fuel ratio sensors. Also called wide-ratio air-fuel (WRAF) sensors and wideband oxygen sensors, this oxygen sensor variant doesn’t generate just two voltage signals (high and low).

Instead, air-fuel ratio sensors alter their signal to directly reflect the oxygen content in the exhaust. They can take more accurate measurements and can even detect very lean air-fuel mixtures, leading to greater fuel efficiency.

What are the Possible Causes of the P0131 Code?

There are various issues that can lead to the PCM logging a code P0131. If you get this code, some of the possible causes include:

  • Issues in the oxygen sensor or air-fuel ratio sensor
  • Defects in the circuit for the oxygen sensor or air-fuel ratio sensor heater
  • Vacuum leaks or a leak in the exhaust system
  • Low fuel pressure
  • Problem with a sensor other sensors (e.g., a bad mass airflow sensor)
  • Problems with the sensor circuit
  • Out-of-date PCM software
  • Faulty PCM

What are the Common Symptoms of the P0131 Code?

The PCM relies on data from the oxygen sensor or air-fuel ratio sensor to regulate the engine’s fuel intake. If a sensor malfunctions, it will disrupt the proper operation of the engine.

Common symptoms attributed to a P0131 code include:

  • Check Engine Light
  • Engine may idle, run rough, or die
  • Decreased fuel economy
  • Increased tailpipe emissions

Occasionally, the vehicle may display no symptoms.

Note: If there are other codes stored in addition to P0131, the causes and symptoms may differ from those listed here.

How to Diagnose the P0131 Code

From a defective air-fuel ratio sensor to issues with the sensor circuit, many problems can log the P0131 code. This can make it tricky to diagnose the problem.

Check out this helpful video reference to find out what the diagnostic process involves:

How to Fix the P0131 Code

Once you’ve established what caused the P0131 code, find out how to resolve it by doing further research online. Look for specific fixes that have proven successful for other vehicle owners who have had the same code and the same underlying cause.

For instance, one of the most common fixes for the P0131 code in a Ford Focus would be to replace the oxygen sensor (provided that the sensor was faulty and had been the one causing the code to set). But that solution may not be as effective for a P0131 in a Toyota, Chevy, or any other vehicle make—especially if what caused the code was a different problem (or set of problems).

If you are new to working on your vehicle, we suggest that you use some of these online auto repair resources and guides to boost your know-how. You may also get an ALLDATA single-vehicle subscription, which will prove extremely helpful for this and all future repairs.

Aside from what triggered the P0131 code, the vehicle’s make and model must also be considered to resolve the issue. This is because automakers may set specific repair instructions that may differ from those you found online (that are applicable to a different vehicle).

Check your owner’s manual before addressing the code to avoid issues.

If you are not confident with your automotive know-how or DIY skills, it may be best to let your mechanic address the DTC.

What Do Oxygen Sensors Do?

O2 sensors have been around since 1975 but became the order of things by the early 80s with feedback fuel systems – and the O2 is the feedback element of the system.

To begin with, Bank 1 is where the #1 spark plug is. Always. Again, determine which bank is which FIRST or you’ll waste a lot of time and effort.

O2 sensor 1/1 is the upstream sensor on Bank 1 (different for a Ford than for a Chevy or a Dodge), and O2 sensor 1/2 is the downstream sensor on Bank 1. O2 2/1 is upstream on Bank 2 and O2 2/2 is downstream on Bank 2. Front wheel drive vehicles can be even more confusing, but when you find cylinder #1 you’ve found Bank 1. If there’s only one bank, that’ll be Bank 1.

Always make sure you know which bank is which before you do any hands-on testing if you don’t want to do unnecessary work or buy unnecessary parts.

mechanic checks and replace car oxygen sensor close up
Always make sure you know which bank is which before you do any hands-on testing.

The inside area of the ceramic zirconia bulb within the conventional O2 sensor is exposed to the atmosphere by way of a small vent where the wires enter the sensor. The outside surface of the bulb is exposed to exhaust gas, and since ceramic zirconia is an electrolyte, the conventional O2 sensor literally creates its own voltage as the O2 levels within the bulb and the O2 in the exhaust fluctuate, providing a 0.2-0.8 volt signal which the ECM/PCM interprets. The O2 sensor reacts very quickly to very minor changes in O2 in the exhaust stream.

The more O2 that is in the exhaust stream, the lower the voltage – the less O2 in the exhaust, the higher the voltage (again, this range is about 0.2-0.8).

With the advent of OBD2, it became evident that the catalyst needed a sensor to monitor its ability to store oxygen – which is what the downstream O2 sensor is there for.

Both the upstream and downstream sensors have an internal heater (sort of like a diesel glow plug but with a bit more resistance), because O2 sensors must be above 600°F to operate. If the heater is burned out, you’ll typically get a code for that, because the ECM/PCM measure current flow through the heater. That being said, if the O2 sensor is running too cold for some reason, it won’t produce sufficient/accurate voltage and the ECM/PCM algorithms are designed to detect that.

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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