Hi,

Got a rough idle on my 97 Mazda 626 (2.0L). Not sure rough idle is the correct term, but the engine jerks once in a while. Also, I noticed some loss of power on highways.

Had a computer diagnostic check and got the ODB II code as follows:

P0421: Warm Up Catalyst Below Threshold (Bank 1)
P0302: Cylinder 2 Misfire Detected
P0171: System Too Lean (Bank 1) or
P0172: System Too Rich (Bank 1)

Talked to the guy who did the test, he wasn't sure if it's P0171 or 172, but must be one of them.

Total cost sums up over a grand! So I decide to try something myself.
Among all these error codes, there exists a possibility that there might be a SINGLE ROOT CAUSE out there, anyone sees what that might be as a starting point?

I will sure check the spark plugs first. Anyone knows how to count the cylinders and banks as indicated in the error code as Cylinder 2 and Bank 1?

Many thanks!