hello all,
I have been having a problem with my 1996 c280 involving its check engine light and odb system and really dont even know where to begin to fix it. I wouldnt worry about it if I didnt live in a state that requires emissions testing.

my problem is that none of the cars emissions modules are set when I hook it up to my code reader. my reader doesnt see any dtc's but when i start the engine with the reader pluged in the check engine lite turns on. If I unplug the reader shut off the car and start it again the cel turns off like normal.

It almost seems as if every time I shut off the car the computer gets reset but I dont know how this would be happening, the car starts strong every day. I think I have checked all the relevant fuses and found none to be bad.

Does anyone here have any idea where i could start to find the problem?

Do you have to go to smog? When you plug the tester into the obd II the Engine light will come on. On this older cars if you don't have to go through smog and the engine runs fine you shouldn't do anything. If you have to go through smog ask for a pre-test and obtain a print out.

That car is a hypochondriac. There are known issues with the OBD systems on the 96-98(?) MBZ- even California (CARB) knows about it. You probably have a drivetrain (not emissions) code causing it, that's why nothing on OBD scanner.

1)- make sure the car is ON but not running when you scan, if you scan while running nothing will appear on scanner but you'll trigger the CEL.

2)- if no codes, then it's a drivetrain code. Disconnect scanner, turn vehicle to ON but don't start car. Let it sit till the light turns off by itself. Drive until it flips on again, rinse and repeat.

You'll probably find some old drivetrain codes flipping the light on, you need the Benz diagnostic to reset everything- OBD scanner won't do it.. It sucks, but that's how it is with these cars. These issues are why I got sick and tired of my C36, and tossed it. It's just how they are.

__________________
03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)

MB used 2 trouble code systems, Analog and digital. Analog can be tested with a LED and a jumper, digital uses CAN Bus and needs a scan tool to access each module.

To make analog HFM-SFI OBD2 compliant, a module marked CARB was used to translate trouble codes from various modules into OBD2 format. this was used for 96 and likely for 95 in some cars.

Here are some CARB box numbers from my previous searching.

018 545 07 32 CARB DIAG MODULE / BOSCH 0 261 200 928

019 545 33 32 CARB DIAG MODULE / BOSCH 0 261 200 938

I know for sure that the 97 SL320 , 97 C280 and 97 E320 are all digital and don't use a CARB box.

Somewhere I came across a states emission manual and it did speak of 95 / 06 MS cars having issues with testing. Perhaps looking at your states rules will help. ( Or look at CA and other states. )