Code P0420

code P0420 may mean that one or more of the following has happened: A damaged or failed oxygen/O2 sensor downstream oxygen sensor wiring damaged or connected improperly. The engine coolant temperature sensor is not working properly damaged or leaking exhaust manifold, catalytic converter, muffler, exhaust pipe. Failed or under performing catalytic converter, retarded spark timing, leaking fuel injector or high fuel pressure cylinder misfire, oil contamination. Not all codes will set a check light, most probably caused by a failed cat converter. i would be looking here first and then if that is okay, continue with the inspection of other components as listed.

How do I check to see what the problem may be? How much driving can I do with this problem? I called my self cleaning my engine which was a mistake got a P0305 code replaced all of the plugs check engine light went off. Now the P0420 code. Should I replace coil on number?

Should I replace number five coil?

may just be moisture shorting out the coil, take off and dry. if the problem started after the wash, may well just be that.