Hello, my 91 740 turbo (411 000 km) has just failed most of the items on the emission test.
Nitrous oxide level was especially bad, carbon monoxide and hydrocarbon levels were a little over the limit.

The car has some blow-by issues as expected by it's age, but no signs of high crankcase pressure and the PCV components were all cleaned out in the last year.

After reading that advanced timing was a possible cause, I found that the timing on my car was about -20 when it should be -12. The best is in good shape and the cam and crank pulleys were found to be properly positioned when at TDC.

The O2 sensor is about 2 months old and voltmeter testing of it was good.
There are no check engine fault codes.

The idle is smooth and steady at 750 and it runs well, which would seem to eliminate an intake air leak.

The catalytic convertor has about 130 000 km on it so I thought to replace it, though I am not aware of a diagnostic test for it.

My car apparently doesn't have a exhaust gas recirc. valve.

Anyone have any thoughts on what else I should check before spending $ on a new cat. convertor? Also, I am stumped about why the timing is advanced.

Allan