01/15/2026
Signs of a Defective Oxygen Sensor and Related DTC Codes
The oxygen sensor (O2) is a key component in the emission control strategy and management of the air-fuel mixture in internal combustion engines.
Its primary function is to monitor the amount of oxygen in the exhaust gases and send this information to the ECU (Engine Control Unit) to adjust the fuel injection in real time. A faulty sensor can alter this balance and cause a series of problems in engine performance.
Symptoms of a Faulty Oxygen Sensor
- Check Engine Light On:
The ECU will detect out-of-range values in the sensor signal and activate a diagnostic code (DTC). This is one of the direct signs of an O2 sensor problem.
- Increase in fuel consumption:
A faulty sensor can generate incorrect readings, leading to a richer mixture than necessary and hence increased fuel consumption. This is because the ECU doesn't receive accurate data to optimize the injection.
- Loss of Engine Performance:
Incorrect air-fuel mixture can cause ignition failure (misfire), unstable slowdown, loss of power, and difficulties in acceleration.
- Smoke on the Exhaust and Elevated Emissions:
A damaged sensor can make the mixture too rich or poor, which can result in excessive emissions of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx). In extreme cases, it can generate black smoke (excess fuel) or white (incomplete burning of the mixture).
Diagnostic Codes (DTC) Related to Oxygen Sensor
DTC codes allow you to identify specific problems in oxygen sensors. Below are some of the most common ones and their meaning:
- P0130: Oxygen Sensor Circuit Malfunction (Bench 1, Sensor 1). It may be due to a damaged sensor or wiring problems.
- P0131: Low voltage on oxygen sensor circuit (Bench 1, Sensor 1). Indicates that the sensor reading is below the expected values, possibly due to contamination or failure of the sensor element.
- P0132: High voltage on the oxygen sensor circuit (Bench 1, Sensor 1). It may be caused by a faulty sensor, short circuit or an overly rich fuel mixture.
- P0133: Slow response of oxygen sensor (Bank 1, Sensor 1). This indicates that the sensor is not reacting to changes in the air-fuel mixture at the expected speed.
- P0134: Lack of activity in oxygen sensor (Bench 1, Sensor 1). This may indicate that the sensor is completely inoperable or that there is a problem in the signal circuit.
- P0135: Oxygen sensor heater circuit failure (Bench 1, Sensor 1). This code suggests that the sensor heating element is not functioning properly, which can delay activation in cold starts.
- P0136: Oxygen Sensor Circuit Malfunction (Bench 1, Sensor 2). Affects the regulation of post-catalyst emissions.
- P0137: Low Voltage on Oxygen Sensor (Bench 1, Sensor 2). It may indicate a fuel mixture that is too poor or a deteriorated sensor.
For a correct diagnosis of a defective oxygen sensor requires the use of an OBD-II diagnostic scanner and an oscilloscope to verify the sensor's output signal in real time.
Additionally, it is crucial to inspect the condition of wiring, connectors, and potential sensor contamination, as elements such as oil, coolant, or unburned fuel can affect performance.
Replacement of a defective oxygen sensor should be carried out with a piece of specifications exactly to those of the original equipment manufacturer (OEM) to ensure compatibility and optimal operation of the injection system and emission control.
