Dreaded CA smog test

[Greenie]

My trusty little green 1991 318IC has failed its first smog test, ever. Passed at 25mph but failed at 15mph due to high NOx and HC. I've never just thrown parts at the car and have maintained it properly. I see from looking on the web this has happened to other M42 engined E30's but no solutions suggested. Is this the cat just wearing out? Just put in new plugs, changed oil, cleaned and opined K&N (in original airbox), new O2 sensor, cleaned out ICV. No fault codes showing. Baffled. Any and all suggestions welcomed!
Car has over 230,000 miles but these are not high load miles, several cross country and up to Canada trips. Doesn't smoke, starts and idles smoothly.
Thanks
Steve

[Balleristic31]

How long did you run the car before taking the test? Properly heating up the cat will make all the difference during a smog test.

[Greenie]

Car and cat were nice and hot. Freeway run before test. Just checked and car is throwing coolant temp sensor error code, but no check engine light. Just had new rad installed, but smog test the same before and after. Is this the engine management system cycling through to get rid of the error code? I think it has to cycle something like 80 times to be erased. I'm going to do a SeaFoam hot soak, maybe two of them, to blow out as much junk as possible from traffic driving and crummy gas, but beyond that I don't know. Could a vacuum leak cause this?
Thanks
Steve

[bmwman91]

How old is the cat?

[DER E30]

Can't you run some kind of alcohol that runs super clean for the test?? Someone confirm?

[benz-tech]

CCCCCCCCAAAAAATTTTTT. I just had to buy another one. Magnaflow 332005 2.25" welds in pretty easily and it gives the CARB guys the reach around. If you do buy one, SAVE the warranty. Mine died 6 months later, which is why I have to get a new one. Being around smog guys at work they me that even a car that runs poorly can be cleaned up by a good cat. Even if it is short-lived.

[Josh1254]

High NOx can be from running lean. Check for vacuum leaks too. Other then that change the cat.

[varg]

High NOx and HC would occur if the engine was running rich and the timing was advanced. I'm not sure if Motronic 1.7 advances timing when the engine is cold, but a bad coolant temp sensor could certainly cause it to run rich.