91 Passat - Fails emissions test...

Good HC and NOx readings, however it fails the CO on both curb idle and the 25mph dyno test...any hints?

Changing out the cat and/or the O2 sensor come to mind, but I hate just throwing parts at a problem without knowing WHY it would be high on just that one aspect of the test.

It's been tuned up (new cap, rotor & plugs, no wires (they're way too expensive anyway) and I plan on replacing the air filter (carefully).

I can't think of anything else that's really emissions related on these machines (that is, there's no OBDII sensor junk or EGR, etc. etc.)

There is a 3mm hex screw hidden under a plug between the fuel distributor & air boot. Usually the plug has been drilled out & someone has adjusted the CO with the screw. Ideally you should have it hooked up to a gas analyzer to reset this. Clockwise richens & CCW leans it out if you want to wing it. You have to be careful not to push on the screw while its running as the arm it is connected to is floating with the air flow plate.

Thanks! I'm new to these machines BTW but if somebody has monkeyed or Mickey Moused with that screw I'll find out soon! I would assume that I'm running too rich now (oxygen starved) hence the high CO...is that the case?

It sounds that way, you would have to have it hooked to a gas analyser to be sure. 1/2 turn CCW would probably lean it out enough to pass, but that's just a guess.

Well, it passed today -- actually it SMOKED the third test with comparable emissions numbers to 2005 models. ZERO CO! If you can believe it. HC and NOx weren't even CLOSE to the limit now!

The problem was a broken wire to the O2 sensor diagnosed with a shop that can read the waveform patterns on a scope. That and retarded timing that was advanced a bit and...away she goes!

A frigging $12.00 wire & mini plug that caused soooo much grief...anyways it's fixed now...

NEVER had to touch that hex screw BTW...

That's good to hear. Some sort of check engine light would have been nice in this case. It's always best not to adjust that screw, but since it seems the plugs have always been drilled out you have to worry about that possibility.