RepairPal is your advocate for car care advice and guidance

Q: Emmissions test fails on 1996 Mercedes-Benz C220

The states emissions test fails due to several not readies on the diagnostic test. I was told to drive the car and come back for a re-test, this has been done several times with the same failed results. The Mercedes dealer said their is no trouble with my car. Is there an on board control unit that I can try to replace or should I take the car back to the dealer for a recheck?
Get a Repair Estimate
Guaranteed by certified locations nationwide. Learn more
RepairPal estimates are guaranteed at over 1,700 quality certified locations nationwide. Learn more
I just went through this with another 1996 Mercedes. I am a Calif. Smog Tech that works at a Gold Shield Station ( means that the State sends us cars that other shops have struck out on ). The Mercedes had several problems. The dealer did, in SF did not even know how to work on the problem The car was broken/malfunctioning for 4 years. What I found is that there is a separate computer that only does the OBD-II monitors. It is called the Diagnostic Monitor or module.The Dealer does not even know where it is in the car because I called several, all over the US, they think it is built in to the main Engine Computer. IT IS NOT!!. The computer is usually in the Engine Fuse Compartment by the round Mercedes Diagnostic Connector. It may even say CARB on it. California Air Resources Board which mandated its existence. The Factory did not even know about it. I had to get it from a special German parts person and it was a used one to boot. However, this definitely fixed the problem. Let me know about this because there are several things to check before putting in this computer, like do you automatic door locks work OK? I will explain why this is so important later.
I am working in Massachusetts and our inspection program is based on readiness monitors as well. These Mercedes apear to be a real problem. I have one at our shop codes have been cleared and as you know that clears all monitors as well. I have driven this vehicles drive cycle several times and not even the 02 heater would run. Is the diagnostic module a common problem with these? as that is what I have been suspecting could be an issue with this car. I believe the module is under the carpet on the pass side mounted to a plate. I am also curious as to the door locks you mentioned. Any information or help would be greatly appreciated.

Where is your shop located? I have a 1996 Mercedes with the same exact problem (OBD-II Monitors not ready). The mechanics and dealers cannot figure it out. Thanks in advance.
I just drain all the gas go to Home Depot and get 2 gallons of alcohol and put it in the tank and it passes every time !!!
Didn't find what you were looking for?