I have a brick that won't pass emissions in Texas. It passes HC and NOX but fails CO. Here are the numbers:

HC(ppm)- Spec/reading - 162/72
CO(%) - Spec/reading - 1.04/1.29
NOX(ppm) - Spec/reading - 576/1163

The tests are run at ~2000 rpm.

Here's the history:
Never failed emissions in the past. '03 reading HC 128, CO 0.34, NOX 974.
Starting running very rich in May 04. AMM replaced. Fixed the problem. Car runs great.
Failed first emissions test in Aug. Replaced O2 sensor, cap, rotor, plugs, FPR, set timing, checked idle switch and temp sensor. I put the O2 sensor on an o-scope and it crosses over from 0.2 to 0.7 volts. It does seem to spend more time above .4 then below, but it does seem to make about 6 to 8 crosses in 10 sec. I can adjust the AMM to get the LED test point to go from "off" to "on/off" to "on". Though I can't see a lot of change in the O2 signal when I adjust it.
Failed second emmision test with almost identical numbers.
I am really stumped.
What is the LED test point indicating?
Could it be possible that the catalytic converter was damaged by the rich running engine? If so, wouldn't the other pollutants have also failed?
Could the replaced AMM be bad?

I could really use the advise of an ECU/emmisions expert to help figure out where to go from here.

Thanks for you help.

John Brady
'86 240 180K

If the engine runs rich but the converter is good, you may fail CO without going over the HC limit because the converter's CO conversion performance degrades more severely than its HC conversion performance as the mixture goes rich. In that situation, NOx would be lower than if you weren't running rich but may still be ok (it seems the spec for NOx is a minimum based on your post) because the converter performance for NOx isn't really affected by running rich (there's just less coming into the converter). I don't know anything about AMMs (haven't had my 740 long enough to need to know yet) - you're concluding the engine isn't running rich based on the O2 voltage - could there be an offset in the base fuel mixture towards running rich?

Zack
1980 245DL M46 273k
1988 740T+ M46 195k

I was dyslexic on the NOX. It should be spec 1163, reading 576. Sorry if it confused anyone.
John

If the engine runs rich, NOx will be lower than it would be if your engine was running with the proper mixture. So low/acceptable NOx, acceptable HC, and high CO sounds like a slightly rich operating condition. Just a guess...