W124 E420 Check Engine - comes on every third start
 

Hello all,

I am seeking your help and collective wisdom figuring out a problem with my check engine light on my 94 E420. I have searched the archives and found lots of info but nothing on point so here goes:

Check engine light has been on for months. 2 different, non MB specialist shops, have claimed to have scanned the codes and come up with a fault they could not recognize with their scan tools.

If I disconnect the battery and reconnect it, the light will be out and generally does not come on until third time I start the car. NO matter the length of time o amount of driving between starts. I checked the LED/built in code reader in the ngine compartment and I am getting 17 flashes (my understanding from an archive posting is that's a general communication fault with some unidentified system). If I hold the button down for 10 seconds or so after the flashes, the check engine light will be off when I restart the car but will come on again after the third start.

I have generally checked the vacuum lines and they appear fine. I have also inspected the wiring harness and that appears to be in good condition also (that was also previously confirmed by the more experienced independent shop.)

I guess the question would be what function on the car self-checks every third start? Anyone have any other ideas? The nearest Benz dealer is difficult to get to and $$$$ and I would like to try to figure this one out for myself. Car is running perfectly and I only want to correct the problem as I am thinking about selling it to buy a W124 wagon.

Thanks.

A lot of the non critical fault codes don't set the C/E light immediately. Some have 2 or more occurances before the C/E light is set. My guess is that you have a consistently occuring problem that sets the light every 3rd occurance. You should take it to a shop with SDS to get a competent diagnosis.

Your car should have a self diagnostic connector with a LED & pushbutton.

Depress the button with the key on for 3 seconds...that blink code will help with diagnosis.

Oops..........you stated code 17......means nothing by itself.

You need a scanner to read the other 4 computers on the CAN network.

1 of those computers hold the key to the problem.

He has determined a code 17 from the DM...

There's always the other LED, lightbulb electrical deprevation, i.e. remove the CE bulb and throw it over your shoulder.

Click here to download my DTC (Diagnostic Trouble Code) guide for your car (1MB, PDF format). As Doc said, you need a scanner, or at least an LED blink code reader, to see what codes are on the other computers. The built-in LED only shows very vague, generic codes from the DM (diagnostic module). Code 17 on the DM means "Data exchange malfunction between individual control modules", which - also as Doc said - means nothing by itself.


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