The oxygen sensor (also called an O2 sensor) is one of the most important elements of the modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensors are located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are a total of two on this SLK - one up stream of the catalytic converter and one after. The sensor located just in front of the catalytic converter measures the mixture of the exhaust gasses exiting the engine. The sensor located after the catalytic converter is used to measure the performance of the converter by comparing the O2 levels before and after. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:
- Irregular idle with warm engine
- Irregular idle during warm-up
- Engine will not accelerate and backfires
- Poor engine performance
- Fuel consumption is high
- Driving performance is weak
- CO concentration at idle is too high or too low
- Check engine light is illuminated
In general, if the oxygen sensor is not working, the car will be running very poorly and will also be outputting a lot of harmful emissions. If the signal received by the computer is out of its normal range, the SLK's computer will almost always give a warning signal that illuminates the check engine light. Sometimes the computer may output an error code stating that the oxygen sensor is reading out of range, when in reality the values are out of range because there is something else wrong with the fuel injection system. Prior to replacing the oxygen sensors, make sure there are no other codes being recorded that may affect the O2 sensor readings.
If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running lean (not enough fuel), and will try to richen the mixture. At the other extreme, if you disconnect the oxygen sensor, and replace it with a small AA battery that supplies 1.5 volts, the ECU will think that the car is running really rich and attempt to adjust the mixture to be leaner.
Needless to say, troubleshooting the complete fuel injection system is beyond this project's scope. If you think that the oxygen sensors may be causing some of your fuel injection problems, they should be replaced. In general, I recommend that you do this every 30,000 miles. You have to jack up the car to gain access to the sensor.
The pre-cat sensor is located in the exhaust pipe just after the manifold. The electronic connector is located on top of the bell housing and the sensor screws into the down pipe. Some people have claimed to have success at changing the sensor from the top of the vehicle, but I have never been able to get my hands in there enough to make it work. Mercedes-Benz makes a special tool for removing the sensor, if you have it, it makes the job a lot easier. If you don't, use a 22mm wrench and remove the sensor from the exhaust pipe. It will be a very tight fit, so take extra care when installing the new sensor not to strip it. When you remove the O2 sensor, you will probably find that it is coated with black soot. This is normal for an old, worn-out O2 sensor.
Install your new sensor snug-tight, or if you have the proper slit-tool and a handy torque wrench, torque it according to factory specs. The new sensors come with anti-seize so there is no need to add any to the threads of the plug before you install it.
Shown here is the connection for the pre-catalytic converter oxygen sensor (red arrow). Simply unplug the connection and unclip the wire from its holder. You can do this from the top of the engine.
From underneath the car, the oxygen sensor is screwed into the exhaust pipe just under where the pipe joins the exhaust manifold (yellow arrow). You will need to unscrew this to remove it.
This is the special tool that you can use to remove the sensor. It is possible to remove it without it but there is not a lot of room so watch your knuckles.
Here is the old sensor removed.
Make sure your new sensor came with a small amount of anti-seize on it. Be careful when installing it not to get fluids or contaminates on the end of the new sensor. Run the wire up towards the top of the bell housing and reconnect it from the top of the car.