Well, my 90 745 just failed emission inspection for NOx. Here's the numbers:

Limit Actual
NOX -- 1227 2968
HC -- 162 100
CO% -- 0.91 0.70

So I'm good on HC and CO, but more than double the NOX limit. What do I have to do to bring the NOX level down? The engine isnt running hot, (new radiator and thermostat within last 6 mths), I cleaned the throttle body over the sumer and the car seems to run fairly smooth.

Any help would be appreciated.
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'90 740 GL Wagon 115K

Yes, I had the same problem with my 91. The catylitic converter was ruined because the oxygen sensor wasn't replaced when it should have been. So they put stuff in the fuel tank to try to get it to pass. You might have to replace the converter and the oxygen sensor. They are both right there together. Mine does not have an EGR valve. So it has to run it's very smoothest. That's why you have to replace the oxygen sensor. It will cost around $275.00.

I'm in NJ and just went through this. I ended up replacing a lot of parts but in the end it was the cat, twice. The previous summer when I was replacing the exhaust, I noticed the the catalyst in the converter was broken loose and disintegrating. I got an after market cat from FCP, but at the time the manufacturer did not differentiate between Bosch and rex regina FI systems, just egr or not. Well, after 5 tries and many parts, Nick at fcpgroton was able to find a manufacturer who makes a cat for b230 engines with the rex regina. With new parts, filters, sensors, etc, I was able to get my Nox down from 3200 to 1600, but never passing. With the new, correct cat, I was able to pass with no problems. It even helped the HC, CO, which were at the limits.

Good Luck,

Tom

The article Sstef points out is a good one.

If it's high NOx and HC and CO are fine, I'd suspect EGR or cat.

But, as per article, it can be O2 sensors too so it's worth getting it checked out to see if it is cycling within the proper range. The cat likes a proper ratio of Ox to function properly.

According to my 1997 test results, a "good reading" for my car (see sig) was 24ppm! That seems mighty low to me. It recorded something about 400ppm.
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Norm Cook Vancouver BC; 1989 745T 200,000KM

Please see this recent thread, the subject comes up fairly often...
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Stef -- 1981 wagon B21A SU M46 318000km

Check Google under "NOX too high". Only 178.000 hits

Yeah, 176K+, but most of them are about power plant emissions! And I know how to solve NOX (and SOX) problems in a power plant -- call my air engineer!
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'90 740 GL Wagon 115K