1988 BMW 528
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I have a 528 BMW 1988 and the car will not pass inspection because the NOX is to high I changed O2 sensor and spark plugs (I used platinum plugs) and still did not pass any sugesstion?

How old is the cat? Knowing how to interpret the emission gases can help diagnose a cat that is ready to be recycled. Is there excess carbon deposited in the combustion chambers? Try a little water injection to reduce carbon build up. Install the stock spark plugs, Bosch W8LCR gapped to .028" Set the valve lash @.010" with the engine cold.

I did change the cat about a year ago

Was the replacement an original equipment cat from BMW?

No it was aftermarket

It is probably a two bed cat and it will not work to remove the NOX as well as the three bed cat that is a stock item. Replace the cat with a original equipment cat and that should do it.