[JohnnyG]

OK, I have a 1991 GT with a bone stock motor with the timing advanced to 14*. My MAC H-Pipe is about 5 years old and came with Hi-Flow Cats. I just replaced the O2 Sensors, cleaned the air filter, and I did a complete tune-up about three months ago.

I failed the Maryland emissions inspection in the areas of Hydrocarbons (HC) and Nitrogen Oxides (NOx) by a very small margin.

The guy at the inspection station told me that it could be due to a number of things... running too rich or too lean... a faulty EGR valve... bad timing... a bad catalytic converter...????

The car seems to run fine... no check engine lights... and plenty of power on tap. Any ideas of what to try first? I only get one free re-inspection and want to nail it on the first try without having to pay somebody to diagnose the problem. Thanks in advance.

John