Wiley Marmot
Well-Known Member
Anything altering the chemical composition of the air mass flowing past intake/exhaust sensors (especially O2 sensors) might very well trigger a CEL; as the sensor is telling the PCM Hey! This air is out of spec!".I did read here someone removed them and got check engine light. I don't see how that would effect it except the fuel/air ratio.
Very common issue with Focus RSs (drove one for 3 years before trading on my GT). Mess with the intake, or worse, the down pipe/cat and you were almost guaranteed a P0420 code because of the altered air mass composition going past the down stream O2 sensor. I had a P0420 caused CEL on my RS's dash for the first 15 months of ownership. Had the catless down pipe swapped out for an OEM catted DP, and Livernois Motorsports adjusted my PCM parameters during a 93 O dyno tune. CEL = gone!
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