My 93 190E 2.6 has a problem with the diagnostic LED located by the battery. When I press the button, the LED does not light up and I have replaced this with a known working unit to see the same result.

If I unplug the battery for a few minutes and plug it back in, the LED starts working with key on engine off. It gives me 1 blink for no error codes. After driving for a few minutes, the car begins to idle high in neutral (~1100rpm) and if I try to read any error codes out, the LED does not work again.

I have replaced the main ECU (011 545 2632) with another unit from the junkyard and it is exhibiting the same behavior. Is there something else I should look at?