Couldn't edit my past post so will make new one.

Problem was failed CAT with high HC CO and NOX. After new CAT HC and CO were no more than 10% of allowable but NOX still 200% of allowable at 15mph and normal at 25mph.

Just one month prior the car had been fully serviced with plugs, ignition wires, timing belt, valve adjustment, compression check, water pump and all new hoses.

This was a California Spec 945T that was randomly chosen for "special inspection" by Air Resources Board when time for biannual smog check came due.

It did need a CAT but NOX remained high. The "Gold Shield" shop (the only type allowed to do these types of inspections and repairs in California) said that there was a crack in the exhaust manifold and it would need to be replaced.

Because they said it would take 2 weeks to get the part I checked for part availability myself and two Volvo dealers and one independent shop I called said that they could get one overnight but wondered why I needed the part since all three places said that in the 17 years these cars had been out there they had not seen an exhaust manifold failure.

Another shop said CAT replacement was needed and when that was done a retest at the "Gold Shield" facility showed everything OK except high NOX at 15mph (1700 RPM).

My son brought the car home and my careful inspection did not reveal any obvious vacuum leaks or loose hoses.

My research did not point to an obvious solution but rather a complex path involving possible malfunctions in O2 sensor, AGR, intake or exhaust leak, coolant temp sensor failure, etc, etc. AGR function test is part of the detailed smog check the car had undergone twice.

Rather than start down that path I contacted a local "Gold Shield" facility and asked if they could check the car. I put in a tank of Chevron 91 octane, a bottle of techron, drove it 40 miles at highway speed and brought it to the facility. The tech looked at the previous two test results and didn't give me too much hope. He placed it on the dyno, ran all the tests and came back saying that it had passed. NOX was now 30% of allowable having fallen from 738 to 179 with nothing being done.

When the tech and I discussed this he asked how hot it had been in the other town on the day of the test. I looked it up and it was 108F. Today on the coast when the car was tested it was about 65F. He wondered if the difference in ambient temp could have made the difference.

What did I learn from this. Only confusion and a bad distrust of the whole emission measurement technology.

However in the future I will make sure the car is tested hot, the weather is cool and will fill up with Chevron and a bottle of techron a day or two before the test. This however I will view as magic since I have difficulty believing that a "brand name" gas and a bottle of joy juice could drop NOX 400%.