My 92 740 Rex/Regina failed nox. 2.50 grams/mile limit, my reading was 2.93 grams/mile. Passed CO and HC. I checked for vacuum leaks, ran injector cleaner thru the fuel system, plugs wires, dist. cap, rotor etc. Got tested again and still failed on Nox and still passed CO and HC. Sooooooo I bite the bullet and put on a new catalytic converter. Get tested again. Now I pass Nox but fail CO and HC. I did nothing else but replace the cat. I reused the same O2 sensor. I'm out of ideas. Anybody?
Mike Kaz
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