Oxygen sensors are one of the many mandatory sensors in modern fuel-injected engines. They are relatively simple devices that check oxygen proportions.
The upstream sensor monitors the exhaust gases before passing through a catalytic converter. Also known as the pre-cat O2 sensor, it gives a snapshot of how complete each combustion cycle is.
- Too much oxygen means the engine is lean and isn’t getting enough fuel
- Too little or no oxygen means the engine is running rich. There’s too much fuel that’s exhausting all the oxygen and still ending up uncombusted.
Another oxygen sensor sits after the catalytic converter. If your vehicle has more than one converter, you will have an oxygen sensor after each catalytic converter. This downstream oxygen sensor provides comparison data for gauging the catalytic converter’s efficiency.
Suppose the downstream oxygen sensor readings are very similar to the upstream sensor’s or fluctuate wildly. In that case, it means the catalytic converter isn’t doing a very good job at scrubbing the exhaust gases.
Why Do Oxygen Sensors Go Bad?
Oxygen sensors can last for very long. They are not a regular service component like spark plugs. However, they will deteriorate fast and get ruined if your engine isn’t working perfectly. Some of the problems that could lead to premature oxygen sensor failure include:
- An excessively rich fuel to air mixture leading to incomplete combustion
- An engine that is burning coolant
- An engine that is burning oil
Also, oxygen sensors can wear out after around 100k miles because they’re constantly exposed to high temperatures. You should expect yours to last for more than double the car’s warranty unless you have any of the above three engine misfortunes.
Signs that You Have a Bad or Failing Oxygen Sensor
A bad or failing oxygen sensor will profoundly impact your engine’s performance and emissions. This makes it an easy problem to infer. Some of the top symptoms to look out for include:
A Check Engine Light
Faulty or bad oxygen sensors will throw a check engine light long before you start noticing the rest of the symptoms. This is because oxygen sensors provide crucial ECU data and can trigger multiple errors if they start going bad.
The top error codes to expect include:
- P0135: oxygen sensor in front of the catalytic converter 1, heating circuit / open
- P0175: System too rich (bank 2)
- P0713: Fuel trim malfunction (bank 2)
- P0171: System too lean (bank 1)
- P0162: O2 sensor circuit malfunction (bank 2, sensor 3)
- P0420: Catalyst system efficiency below threshold bank 1
Some O2 sensor-specific errors
- P0130: 02 Sensor Circuit Malfunction (Bank I Sensor 1)
- P0131: 02 Sensor Circuit Low Voltage (Bank I Sensor I)
- P0132: 02 Sensor Circuit High Voltage (Bank I Sensor 1)
- P0133: 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)
- P0134: 02 Sensor Circuit No Activity Detected (Bank I Sensor 1)
- P014C: O2 Sensor Slow Response – Rich to Lean (Bank 1 Sensor 1)
- P014D: O2 Sensor Slow Response – Lean to Rich (Bank 1 Sensor 1)
- P014E: O2 Sensor Slow Response – Rich to Lean (Bank 2 Sensor 1)
- P014F: O2 Sensor Slow Response – Lean to Rich (Bank 2 Sensor 1)
- P0150: 02 Sensor Circuit Malfunction (Bank 2 Sensor 1)
- P0151: 02 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
- P0152: 02 Sensor Circuit High Voltage (Bank 2 Sensor 1)
- P0153: 02 Sensor Circuit Slow Response (Bank 2 Sensor 1)
- P0154: 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
- P015A: O2 Sensor Delayed Response – Rich to Lean (Bank 1 Sensor 1)
- P015B: O2 Sensor Delayed Response – Lean to Rich (Bank 1 Sensor 1)
- P015C: O2 Sensor Delayed Response – Rich to Lean (Bank 2 Sensor 1)
- P015D: O2 Sensor Delayed Response – Lean to Rich (Bank 2 Sensor 1)
- P0171 / P0174: O2 Sensor has detected a Lean Exhaust Bank 1 / Bank 2
- P0172 / P0175: O2 Sensor has detected a Rich Exhaust Bank 1 / Bank 2
Remember that these error codes don’t directly mean that the oxygen sensor is the problem. You have to dig deeper or get the help of a mechanic to ensure that you are not misdiagnosing your engine.
Bad Gas Mileage and Rotten Egg Smell
Since the oxygen sensor plays a vital role in controlling fuel delivery in an engine, faulty readings could throw off how much fuel injectors introduce into the system. When the problem leads to too much fuel injected, your gas mileage will suffer.
Apart from worse gas mileage, you will also get a rotten egg smell at the exhaust pipe or even black smoke if the uncombusted fuel’s soot is too much for your catalytic converter or Diesel Particulate Filter to handle.
Rough Idle and Misfires
Anything that affects the air to fuel ratio will affect how smooth and powerful your engine can be. It will start running rough and even misfire if the deviation is too much. You will detect more vibrations at idle, loss of power when accelerating, stalling, and hesitation.
How to Troubleshoot an 02 Sensor
Luckily, modern vehicles give us access to so much sensor data without having to break a single wrench or unplug any sensor. All you need is a good OBD2 scanner that can pull live data going to the ECU and other control modules.
Live data helps you dig deeper and find out the true cause of the check engine light instead of guessing and fixing anything related to the light.
How to Test an Oxygen Sensor Using an OBDII Scan Tool
I assume you have an OBD2 scan tool that can read live data. If you don’t, you can find a friend with one or buy yours here on Amazon.
- Identify the OBD2 diagnostic link connector in your vehicle and plug in the scan tool. Consult your vehicle’s manual or search online for the connector’s location if you don’t know
- Turn the vehicle on so that the scanner can communicate with the ECU
- Follow the connection instructions for your OBDII scan tool to establish a connection. This will vary depending on scan tool type and if it is Bluetooth operated or not
- Wait for the engine to warm up
- Go to the live data section of your scan tool and find the 02 sensor readings.
Once you have identified the 02 sensor readings, it is time to check if they are within expected parameters.
While this will vary from vehicle to vehicle, here are the general expectations
For the Upstream Oxygen Sensor
The sensor should steadily fluctuate between a lean reading and a rich reading. If your scan tool maps the data, this will result in a fairly steady wave-like graph. The readings will vary from 0.1V for lean and 0.9 for reach.
Any readings within this range mean that your upstream oxygen sensor is in good working condition.
If it sticks to just one side ( rich or lean side), try changing a variable in the air/fuel ratio and see if it makes a difference. This test is simpler if it is stuck on rich. Introduce a vacuum leak to see if the engine will stop running too rich. If there is a fluctuation, then the sensor is OK, and the problem is somewhere else.
ProTip: Oxygen sensors often fail on rich bias by shifting up to the rich side of the scale. This makes the vacuum leak introduction useful in testing sensors stuck on rich.
Note that a very high voltage reading doesn’t necessarily mean the sensor is damaged or the engine is running rich. It could mean your EGR valve is clogged or faulty.
Other crucial things to check include:
- Identify the oxygen sensor heater wires and confirm that they are receiving the 12V power
- Confirm that every plug is firmly attached and the oxygen sensor contacts are not dirty
- Confirm that your sensor has good ground and isn’t stuck to the faulty ground signal (varies from manufacturer to manufacturer)
ProTip: Sometimes, troubleshooting an oxygen sensor can take more time than it is worth. This is especially so if your mechanic charges per hour or you are handy enough to replace an oxygen sensor yourself.
For the Downstream Sensor
The downstream sensor readings shouldn’t fluctuate as much. A steady readout shows that the catalytic converter is doing a great job scrubbing exhaust gases. Wild swings beyond acceptable thresholds throw the ‘Catalytic Converter Efficiency Below Threshold’ error and a check engine light.
How Often Should I Replace My Oxygen Sensors
The recommended change interval varies depending on your vehicle model and manufacturer. The most common interval is replacing the oxygen sensor between 60,000 and 90,000 miles. Many people rarely get to it since they sell their vehicles long before this.
Changing oxygen sensors should be on your to-do list if you buy or own a high mileage vehicle.
How Much Does it Cost to Change an Oxygen Sensor?
Expect to spend an average of $60 to $500 on a replacement depending on your vehicle make, model, and prevailing labor costs.
Depending on how much your mechanic charges per hour or per job, you can spend between $20 and $200 in parts and between $0 (if you DIY) and $200 in labor.