Oxygen sensors are an extremely important part of your BMW E36. Unfortunately, as our E36s reach into the higher mileage oxygen sensors become one of the most common parts needing replacement. I have noticed that on BMW E36s, oxygen sensors tend to go bad like clockwork around the 100,000-mile mark. Depending on the model and motor you have, you may have between two and four oxygen sensors.
In this article, I will discuss what BMW E36 oxygen sensors do, why they are important, the symptoms of a bad oxygen sensor, and how you can replace them in under one hour.
If you would like to see a video DIY on replacing BMW E36 oxygen sensors, there will be one at the bottom.
What does an oxygen sensor do?
All vehicles manufactured post-1980 will have oxygen sensors somewhere on the exhaust manifold. The 02 sensor works by producing a signal that is sent to the ECU of the fuel injection system. The ECU adjusts according to the signals received from the oxygen sensor.
A properly working 02 sensor keeps your emissions in check and lets you know when emissions are too excessive. A faulty 02 sensor will usually cause a check engine light, along with performance issues. A vehicle with a bad 02 sensor usually will not pass inspection.
Every BMW E36 will have at least two 02 sensors, one in front of the catalytic converter (upstream), and one behind the catalytic converter (downstream). My little brother’s E36 318i for example has a total of two oxygen sensors, however, my six-cylinder 328i has four oxygen sensors.
The ideal ratio for oxygen to fuel is 14.7:1, this may vary slightly. If there is too little oxygen, fuel will be left after combustion and this is known as a rich mixture. Whereas if there is too much oxygen present, this is referred to as a lean mixture. When an oxygen sensor fails to function properly, the computer must guess how much gasoline to use, and this will result in decreased performance.
Symptoms of a bad BMW oxygen sensor
The most obvious signs of bad 02 sensors is a check engine light and a noticeable decline in fuel mileage. Bad 02 sensors can also cause a loss of power, I have always been able to locate BMW 02 failures because on all three E36’s I’ve replaced them on they went out at around the same mileage. Anywhere between 100xxx – 120xxx the 02s will begin going out, and if one goes bad its best to go ahead and replace all of them.
Other common symptoms of bad oxygen sensors include:
- Poor engine performance
- Rough idle during warm-up
- Engine backfiring
Another common symptom is increased emissions/smog coming out of the tailpipe. Oxygen sensors generally go bad over time, and as they get worse you will likely notice more and more smoke coming from the rear-end of the car.
Cost to replace oxygen sensors
Oxygen sensors are somewhat expensive, BMW E36 oxygen sensors cost approximately $100 each. Installing them yourself could save you hundreds of dollars in labor. These sensors are typically not very difficult to replace, however, you will need jack stands and/or a ramp as most of the work is done underneath the vehicle.
You could also go cheap with the oxygen sensors. I have seen some aftermarket E36 sensors going for less than $50 apiece, but I would strongly suggest you stay away from these. The OEM oxygen sensors are of high quality and should last the remainder of the life of the car. If you buy a cheap aftermarket you might end up replacing them again in 30,000 miles.
Besides the fact that bad e36 oxygen sensors will cause a performance and gas mileage decline, I strongly urge anyone that suspects they have a bad O2 Sensor to replace it as soon as possible. This is because bad upstream sensors can make the catalytic converter work harder and shorten its lifespan. Converters are extremely expensive to replace on a BMW.
My brother’s 318i 02 sensors went bad, but he did not want to shell out the $200 for new ones so he continued to drive on the old sensors for several months. Lo and behold, his catalytic converter went bad, and the $200 bill suddenly became $700. The moral of the story is DO NOT WAIT TO REPLACE YOUR OXYGEN SENSORS!
BMW E36 oxygen sensor replacement procedure
- 22 mm wrench/22 mm oxygen sensor socket
- Jack & jack stands
- Oxygen sensor (preferentially all of them, most models have 2, 328i & M3 will usually have 4)
- Small flat-head screwdriver
- Jack up the front end of the vehicle and set the front on jack stands. If you have four jack stands or some thick pieces of spare wood hanging around you should also jack up the rear end.
- Oxygen sensors are one of the components that BMW made it really simple and quick to replace on the E36 Series. Once you’re under the vehicle you should be able to see both the downstream and upstream oxygen sensors. (On the 318i, one of the sensors was more difficult to reach, but it can still be done without the use of a special tool)
- At this point, you should disconnect the oxygen sensor connector, and then remove the plastic cover on the oxygen sensor wire trailing back to the oxygen sensor. This can be achieved quickly with a small flat head screwdriver.
- Take your 22 mm wrench or socket and remove the oxygen sensor. It will probably take quite a bit of force initially. More than likely that sensor has been on there for more than twenty years. (If the sensor won’t budge, sometimes I use a little bit of PB Blaster or Liquid Wrench to loosen the sensor. If you go this route be sure to give it about thirty minutes).
- Remove the electrical plug from the wiring harness. Rotate the plastic retainer clip clockwise to pop it out of the harness.
- Once the old sensors are removed simply install the new and tighten the sensor.
Oxygen Sensor Replacement Notes
- Typically the new sensors will have the correct plugs but they often have significantly more wire than you need. That is okay, I just wrap up the extra wire and zip tie it somewhere along the frame.
- It is a really good idea to add some anti-seize compound to the threads on the oxygen sensors. Hopefully, you will never have to replace these sensors again, but in case you do, anti-seize will make them easier to remove next time.
- Take a look at the old plug after you have removed it. It will probably be covered in black soot and will possibly have some white oxidation on it.