Hi all! Great forum. I was wondering if anyone here knew a thing about emission testing. I have an otherwise awesome 95' Civic that failed emission testing. Specifically, it failed the NOX part of the test on the lower RPM; HC and CO was okay. The margin of failure wasn't much.

Allowed NOX level: 1372 ppm
Actual NOX level: 1478 ppm

I heard you should test when it is warm. Is this true or does anyone have any suggestions for what is causing that kind of reading? Your help is greatly appreciated.

 

I had the same problem. The high NOX readings are due to high combustion chamber temps. These can be cause by several things from a google search:

Misfire condition
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Corroded or damaged engine sensor electrical connections

Here is what I did. I replaced the cat, and the O2 sensor since my car doesn’t have an EGR. That didn’t fix it. The car was running fine but it would get hot from time to time. It turned out to be a small leak in the head gasket that would cause the high temps in the combustion chamber. You might want to get a compression check done on your engine.

Oh and yes you must test it when the car is warm. Never test the emissions on a cold car, the O2 sensor only operates correctly when it is brought up to the proper temp.

Good luck.

 

drive it hard on the way to the testing place, then leave it running while you go inside. make sure it stays running right up until the test.

 

The hotter your catalytic converter is, the better.

 

im so glad i dont have that... hey, there is a good thing about maine!