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However, how could any spark plug problem in one cylinder cause that kind of NOx readings with no HC and no CO?
The spark plugs are at most an indication of some other issue in this case, in my opinion. Perhaps that's what you meant.
Granted, not likely the specific problem.
What I meant was that I've spent literally days and days trying to get some of these cars to pass California smog. And like most automotive repairs, the problem is usually something simple.....but you might not find it until the second day is over.
Trust me, I've had one of everything you could ever imagine make these vehicles not pass a smog inspection, here.
When it comes to problems passing smog, you certainly do not want to miss any low hanging fruit and then move onto what are more obscure problems.
And a green spark plug and spark plug connector are about as low as the fruit hangs....
I did too today It must be a good day to have a 928 take the smog test. My 81 is running lean with Bosch Platinum +4's is it and a bad fuel pump. I thought it would fail due to high NOx.
This is great that you passed after so much trouble