Installing a new CO2 sensor

As my Mercedes Benz still had the original CO2 sensor installed and with a milage of nearly 200.000km, it was time to remove it and install a new one. The normal life expectancy is between 100.000km and 120.000km. As the sensor is installed for so long in the exhaust pipe and very often rusty and baked with the pipe togother, it is very tricky to get it loose. Sometimes it is so bad, that not even heating up will get the bugger out. The only option is then to install a new piece of exhaust pipe to be bale to install a new sensor.


Bosch CO2 sensor 0 258 003 146


22mm closed wrench
hammer, phillips srew driver, cable cutter, copper paste

How to

The electrical connection of the sensor is in the passenger foot well area. Romove the carpet and locate both connectors at the floor next to the tunnel. Disconnect them. To reach the rubber seal, remove the little phillips screw in the tunnel cover, move the cover a bit that you can reach and look under it. You should be able to locate the rubber seal.

Locate the sensor and cut the wire. Remove the heat shield for better access. As the cable is cut, you can use the closed wrench to loosen the sensor. I did start up the car and left it running for a minute to slightly warm the pipe. Put the wrench over the sensor, hold the wrench tight, do not slip and tap hard but carefully a few times with the hammer against the wrench. I was lucky, as 4-5 times tapping the sensor got loose. Remove the sensor. The new sensor comes with copper paste on the threads, but to be sure to get it open as good as this time, I added carefully some copper paste to the threads and inserted the sensor. Make it tight.
Remove the old cable and the rubber seal, if not done already. Put the plugs through the hole and close off with the rubber seal. Tighten the cable underneath like the old one. Install the heat shield.

Now in the passenger foot well, place the cable behind the tunnel cover, connect the plugs, tighten the tunnel cover and put the floor mat back. If your idle was not proper due to a bad CO2 sensor, it should now be nice and smooth.