Symptoms of a bad oxygen sensor
- Poor fuel efficiency
- Sulfur or rotten egg smell from exhaust
- Black smoke from the exhaust
- High emission levels when smogging vehicle
- Engine hesitates, skips, or sputters under load.
The oxygen sensor or O2 sensor for short is located on the exhaust before the catalytic converter. The O2 sensor monitors the oxygen content of the exhaust gas stream. This measurement is read by the ECU and it changes the air-fuel mix ratio by altering the pulse width of the injectors. 14.7 parts air per 1 part fuel is the ideal mixture ratio to minimize exhaust emissions. This is why the O2 sensor is important to the operation of your vehicle.
Why am I testing the O2 sensor?
I am testing the O2 sensor to rule it out while diagnosing excessive fuel consumption in my E28. I have tested all of the components in the engine bay that can cause this issue, the O2 sensor is next before the fuel filter and fuel pump.
- Check for voltage at the O2 sensor electrical connection
- Test the O2 sensor heater
- Check the voltage output of the O2 sensor at the operating temperature
- Voltage reading at Idle= 0-0.4V
- Voltage reading under load= 0.5-1.0V
- Check the heater sensor relay
- Tools/ Parts
- Safely jack up the front end of your E28. I recommend using an Angry Ass jack pad to jack up your vehicle from the Stock jacking point.
2. Locate the O2 sensor and electric connection, and bring your vehicle to the operating temperature which is 180°F.
3. O2 sensors come in 2,3, and 4-wire connections, use the diagram below to identify the proper terminal to probe.
4. While the vehicle is idling, disconnect the electrical connection and introduce the positive probe to the output signal terminal. My E28 is equipped with a three-cable sensor and the output signal pin is #1.
5. Increase and decrease the engine speed and monitor the voltage. You should see a voltage increase to 0.5-1.0V when accelerating, and a voltage decrease to 0-0.4V at idle. I revved the car a little high, it’s not recommended to rev it too high. Another option is to drive the car safely while the connection is probed.
6. Check that there is the voltage at the O2 sensor heater pin, you should measure 12V with the switch on. If the reading is below 12V or has no voltage check the O2 sensor heater relay.
7. If the O2 sensor fails any of these tests it requires replacement.
My O2 sensor failed the Idle voltage and voltage under load, I will replace it and perform the test again. I recommend if the car is drivable probe the connector and drive the vehicle back and forth on your driveway to measure the voltage under load. It is not recommended on automatic E28 to rev the engine too high as it can damage the transmission.
I had some time to get the O2 sensor swapped out today, if you replace your O2 sensor you can remove the sensor with the entire harness and work at your toolbox comfortably.
- Before cutting and splicing the cables line up both the old and new O2 sensors to see the order of the cables to properly splice them together.
2. I used the shrink tubing from Harbor Freight and a heat gun to protect the cables I spliced.
3. Apply anti-seize on the threads of the O2 sensor, this will make it easy to remove the O2 sensor next time.
I filled up the fuel tank and will track the mileage for any changes.
I filled up the car with 12 gallons of fuel and drove it around for a couple days while tracking the miles driven. With the 12 gallons I drove 204 miles divided by the 12 gallons pumped I got 17 miles per gallon. Compared to the 13.4 mpg I was gettign before the O2 sensor was replaced its a small improvement. I am still waiting for the new injector to arrive to be able to remove all injectors and rebuild 5 and replace one. Will fill up and test again once that is done.