A bad O2 sensor can have symptoms like a check engine light or poor gas mileage. Find out the signs of bad oxygen sensors, how to find and replace them yourself, and more.
What Is an Oxygen (O2) Sensor?
Oxygen (O2) sensors, also known as a lambda or air-fuel ratio sensors, monitor oxygen in the exhaust to determine the air/fuel ratio, or how rich or lean the engine is running. They send this information to your vehicle’s computer so it can adjust the air/fuel ratio. Essentially, O2 sensors are there to help the engine produce as few exhaust emissions as possible and to help it perform optimally.
What Are the Symptoms of a Bad O2 Sensor?
Ideally oxygen sensors will last for around 80,000 miles or more. These are some symptoms that can arise with a bad oxygen sensor.
Poor Gas Mileage
If the oxygen sensor is malfunctioning, the computer may have trouble adjusting the air/fuel ratio, causing you to waste fuel and experience bad gas mileage.
Rough Engine Idle
If the computer has trouble making adjustments to run optimally from a bad O2 sensor, it can decrease engine performance and cause a rough running engine when idling.
An engine trying but failing to run at its best from a bad O2 sensor can cause engine surging, meaning the RPMs fluctuate up and down when idling.
Check Engine Light
Often, defective oxygen sensors will turn on the check engine light. Here are some common OBD-II codes related to O2 sensors:
- P0130 – Bank 1 Sensor 1 Circuit Malfunction
- P0131 – Bank 1 Sensor 1 Circuit Low Voltage
- P0132 – Bank 1 Sensor 1 No Activity
- P0133 – Bank 1 Sensor 1 Slow Resistance
- P0134 – Bank 1 Sensor 1 Sensor Circuit No Activity
- P0135 – Bank 1 Sensor 1 Sensor Heater Circuit Malfunction
- P0136 – Bank 1 Sensor 2 Sensor Circuit Malfunction
- P0137 – Bank 1 Sensor 2 Sensor Circuit Malfunction
- P0138 – Bank 1 Sensor 2 Sensor Circuit High Voltage
- P0139 – Bank 1 Sensor 2 Sensor Circuit Slow Response
- P0140 – Bank 1 Sensor 1 Circuit No Activity
- P0141 – Bank 1 Sensor 2 Sensor Heater Circuit Malfunction
- P0150 – Bank 2 Sensor 1 Sensor Circuit Malfunction
- P0151 – Bank 2 Sensor 1 Circuit Low Voltage
- P0152 – Bank 2 Sensor 1 Circuit High Voltage
- P0153 – Bank 2 Sensor 1 Sensor Circuit Slow Response
- P0154 – Bank 2 Sensor 1 No Activity
- P0155 – Bank 2 Sensor 1 Sensor Heater Circuit Malfunction
- P0156 – Bank 2 Sensor 2 Circuit Malfunction
- P0157 – Bank 2 Sensor 2 Circuit Low Voltage
- P0158 – Bank 2 Sensor 2 Circuit High Voltage
- P0159 – Bank 2 Sensor 2 Sensor Circuit Slow Response
- P0160 – Bank 2 Sensor 2 No Activity
- P0161 – Bank 2 Sensor 2 Sensor Heater Circuit Malfunction
“Sensor 1” refers to “upstream” sensors, and “Sensor 2” refers to “downstream” sensors. The bank location can be found if you know where cylinder 1 is on your engine. Bank 1 will always be on the side of cylinder 1, and bank 2 will always be on the side of cylinder 2.
Where Are O2 Sensors Located?
The location and amount of oxygen sensors in your car will depend on the year, make, and model. Generally, you’ll find them mounted along the exhaust system, with upstream O2 sensors placed on the exhaust manifold or front exhaust pipe and downstream O2 sensors placed after the catalytic converter.
More on upstream vs downstream O2 sensors and how to find bank 1 or 2
How to Replace an Oxygen Sensor Yourself
Learn how to replace the oxygen sensor yourself with step-by-step instructions from our experienced mechanics. You can find how-to videos related to your model for this and more repairs in our how-to video library.