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P0420 Diagnosis

999K views 3K replies 324 participants last post by  bluehawk91 
so, I had 0420 for over a year, finally got a mechanic to replace the forward sensor, and 0420 has stopped. Now I have knocing, pinging, code 0171B code 0174B. Put in a new MAF... now car won't idle. got a guess what my next problem might be?
 
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