My Dodge Caravan 3.3l (270,000 klics) has passed aircare but with nox reading that are 50% above average. I'd like to adjust one thing at a time an retest. What is the most likely cause.
- dirty air filter (never replaced)
- spark plugs (never replaced)
- EGR valve (is this ever the problem)
- O2 sensor etc.
Thanks
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