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97 Monte Carlo PO305 (cyl 5 misfire) and PO420 (Catalyst system efficiency below threshold)?
Hello, I have a 1997 Chevy Monte Carlo LS 3.1L. Recently it has been sluggish and non-responsive on the throttle. When scanned, I get a PO305 and PO420 code. Before I scanned, I changed out all the spark plugs with bosch platniums +2 (To find out they aren't very good afterwards via peoples opinions) and I changed the O2 sensor (Tightened with a cresent wrench, but should be in there good enough)
I'm just wondering if this is one of those problems that some mechanic has seen 100 times, and has a good idea of whats wrong. I've been trying to get it fixed, but don't want to drop too much money on something that may not even be needed.
What would be your approach to figuring it out? thanks!
Cleared the codes, put some slick 50 gas treatment in (for giggles), and went a bit heavier on the gas pedal. It drove fine until I shut it off, turned it back on, and check engine light was back on. two instances of PO440, w/ nothing else. After that the car was very sluggish.