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Forum > Failed Smog Test > Question

2000 S430 Mercedes Benz Check Engine Light Does Not Turn ON

Car I bought failed SMOG TEST ONLY due to a Non-operable CHECK ENGINE LIGHT. They said it should come on when key is turned and then go off, but that my cars light doesn't go on. Also, it gave Fault Codes: P0400, P0422, P0432, P0305, P0306, P0301, P0302, P0303, P0300. Overall test results: Visual-PASS, Functional-FAILED and Emissions-PASSED. I took it to mercedes mechanic and he suggested replacing plug wires and vaccum hose but he COULD NOT guarantee that it would then pass smog test. He didn't have any suggestion about why or how to get check engine light working. It was suggested that I try running fuel injector cleaner through gas tank as I have bought fuel from several different gas stations and that I may have received bad fuel. The mechanic I took it to is the same mechanic previous owner used. I am at a loss as to what to do now.
Answer:

A Mercedes Benz certified mechanic (preferably a smog check technician) can use the MBZ diagnostic scanner to manually turn on the check engine light. If they attempt this and the light still won't illuminate, chances are the light bulb is burnt and will need replacement; this is the best case scenario. As far as the trouble codes... trouble codes P0300 through P0306 are misfire related. They were probably all set at the same time when your Mercedes's engine possibly began to run rough, even for a short period of time. We recommend resetting the engine control computer and see if the codes reappear. This can be done using the MBZ diagnostic scanner as well. Trouble codes P0422, and P0432 are CAT Efficiency codes. These codes indicate your Mercedes might have catalytic converter problems or possibly the rear oxygen sensors are failing. Trouble code P0400 relates to the EGR system. It is possible the EGR valve is plugged up or defective. In the best case scenario trouble code P0400 may have been set while your car's engine was not performing well enough to provide the vacuum needed to open the EGR valve; resulting in the computer thinking something is wrong with the valve itself. We recommend first fixing the check engine light problem and resetting the ECU (clearing all the trouble codes). From there you can drive your MBZ and see which if any trouble codes return. posted by SmogTips Support

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