P0302 P0420 P0170 Mazda 626 Not sure what should be first step

Question

I'm looking for a common denominator between these 3 codes and it looks like the P0170 (fuel trim) could be caused by the P0302 (cylinder#2 misfire)

But not sure if the P0420 (catylytic) could be contributing as well or completely separate issue.

Trying to figure which is the first place to start (and the least expensive)

Thanks,
Chris

Answer 1

P0170 Fuel trim malfunction bank 1

P0302 Misfire number 2 cylinder

P0420 Catalytst effiency below threshold

A vacuum leak can contribute to the misfire, any misfire or vacuum leak will make this engine run lean in doing so the catalytic will be below its threshold. MAF (mass airflow ) airduct that has a torn duct can cause a lean fuel trim with a misfire,and give  P0420,a slightly open EGR  will do the same.