I just failed the DC Nox testing. I've got a 1988 240 with about 130K, mostly original parts, well maintained (or so I thought). I had a reading of 3.85 NoX grams/mile, 3 was the acceptable mark. I have not seen testings based on this grams/mile scale. Does anyone have an idea if this is close or so far off that I've got to replace the catalytic converter?

I don't drive the car much and need to get an oil replacement. Does anyone think an oil change and/or an additive, better gas or anything else other than major tinkering (which I'm not great at) would help it pass? Otherwise, the HC and CO readings were excellent: HC was .4 grams/mile with 2 being standard and CO was 5.18 grams/mile with 30 being standard.

One horrible thing about DC emissions: you can only get tested at one place and frequently have to keep the car in idle for upwards of an hour while waiting. (all that and one of the highest tax rates in the country!) I'm sure that can't help the engine temp but would it be better to stop and restart or just keep the engine running? Any thoughts greatly appreciated!
Jeff