No Check Engine Light - can't get smog

[dekota]

I have a 1991 Q45. The check engine light was lighting up, ended up being the MAF. I replaced it with a used one, cleaned the throttle body, cleaned the EGR and the passage (it was totally blocked), and replaced the a cracked plug on the TPS.

Everything was working good...but then I went for my dreaded smog test, and it failed becasue the "check engine light" would not come on when the ignition is first turned on.

I pulled the instrument cluster, and checked the bulb, it is fine. With the engine running, I pulled the hose on the EGR (which should throw a check engine light) - nothing.

Any suggestions on what to explore next?

[dekota]

Well, I am replying the solution to my own problem.I checked out the ECU, it is supposed to have an LED that lights at the same time as the check engine light. It did not. There is a little screw on the side of the ECU that is used to access the test mode - ends up that that screw was not turned fully counterclockwise (like it should be for the run mode). I turned it fully counterclockwise, and walla, check engine light worked, and LED on ECU worked. Not sure how it got like that, I have had the car for about 130K miles and have never fiddled with that thing.

I went straight to the smog shop, got a test, and passed. The emissions were even improved!

[lino]

Congrats! . Lucky it was a "free of charge" fix. Someone must have not set it back to the proper position last time they were doing some testing.