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I have a PO421 code (warm up catalyst efficiency below threshold bank 1). Does anyone know what part/s would need to be replaced to fix it? I think it could be the catalytic converter, but it may be something simpler (O2 sensor?).
Any help would be appreciated. Thanks, Rich


What's the mileage and are you smoking from a valve guide seal issue? Perhaps the cat's plugged if you've been burning oil but honestly can't be sure. Hopefully someone else can chime in with some better ideas.