I took my 1993 940 in for an emission test it has about 250,000 km. Doesn't use much oil, 1 liter in a recent 10,000 KM highway trip.

It Failed!
LIMIT Reading Result
HC 58 56 Pass
CO .32 .67 Fail
NO 435 573 Fail

Changed plugs, rotor, cap, air cleaner, oil, filter, O2 Sensor, Cat Converter.

HC 58 08 Pass
CO .32 .00 Pass
No 435 860 FAIL! Plus an increase from before of 287 or so. The first test was only 138 out of range now I am 325 over a huge increase. After a 10,000 km road trip and an oil and filter change I am at NOX of 928 a bit higher than before the trip. The new Cat was from Partsource as were all the other parts. I am getting 12.55 KM/Liter have lots of power, good starting and no hesitation. What can I check next. When the engine light comes on I get a clear 1 1 1 reading on #6 plug on #2 I read 2 3 2 and 1 3 2 . Battery voltage and manifold or fule pressure. ike

They told me to let the cat settle in for a week or two after a few days the NOX was a little better

 

Anything would just be a shot in the dark. pay the $80 for a diagnostic. You'll probably end up saving money.

 

So I just got mine back from being smogged and found out that when I changed my 02 sensor, I didnt reinstall the exhaust pipe properly, leaving a leak.

I'm glad I paid for a diagnostic, which was a flat fee. They found the exhaust leak, found a bad catalyst and a few other things.

Now my car is running great and passed smog with flying colors.

 

First thing to ck is what DTCs are telling you (from here):

1-3-2 Battery voltage too low/high Poor battery or charging system

2-3-2* Fuel trim (lambda control) too lean or too rich at idle }If too rich: high fuel press., leaking injector } various driving problems Fuel trim (lambda control) too lean at idle Oxygen sensor


So ck batt and charging voltage first; low batt voltage can cause all kinds of problems/gremlins.

What kind of O2 sensor did you install? If it's not OEM that most likely is a problem.

Assuming those exhaust readings above were at idle?

A good info link on the topic of exhaust gas analysis is here:
http://www.autoshop101.com/forms/h56.pdf

At the end of the day I would agree w/IRMB; to effectively diagnose something like this you need to know what you're doing and fairly extensive set of diag tools. At min going to need to get exhaust sniffer plugged in ahead of the cat to analyze what's going with combustion.

If I were to guess (and this is total WAG) am thinking EGR valve may be leaking at idle (carboned up) and/or faulty O2 sensor.

Good luck and let us know what you find.

 

I found out that they had given me the wrong )2 Sensor and also the Cat. Replaced them both and now my NOX is still too high, higher than when I started, no codes at all and great gas mileage like 36 MPG canadian. It's 12.64 KM per Liter. The exhaust manifold seems very hot the cat is 250 pre and 350 Post. No leaks in the pipe