I would try doing a few things.

1. check the spark plugs and replace with original bosch standard copper core plugs if needed.
2. Run the tank until almost empy, then run some fuel injector cleaner through it with a 1/4 tank of premium and run that until it is gone and then fill up again with premium.
- you can also try to find some stuff called bg44k which supposedly will give the engine a very good cleaning. I myself have not used it, but do some searching on the forums.
3. Drive the car for at least 20 - 30 minutes at high rpm's (also known as an italian tuneup) hopefully on a freeway in like 3rd gear to get the cat nice and hot. then take it in for the test, and don't let the car sit and cool down for too long before it is tested.
4. Make sure you have changed the oil and filter and that it is nice and clean. also check your air filter and make sure it is not dirty or somewhat clogged up.


My car always fails if it has sat too long before they get to testing it. but the place I go too will always run it again right afterwards and it always passes then. the guy even told me these cars need to be really warmed up and hot. When it is in that state, it always passes with flying colors well below the max levels.

If all else fails, try some of the guaranteed to pass stuff they sell at auto parts places.

Definately search the forums, because this topic has also been covered quite a bit.

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U