Error code P0421- wouldn't start, now is starting


Asked by Jul 19, 2015 at 07:47 PM about the 2003 Audi A4 3.0 quattro Sedan AWD

Question type: Maintenance & Repair

Audi A4 Quattro 3.0 virtually out of gas last night. drove to store fine. wouldn't start after
shopping. was raining, not sure that mattered. Put in gas last night, no start. Dash lights
went on, no noise when turn key. Today has been starting fine. clock had stopped and
now has started again. getting error code P0421. Likely causes/fixes? thanks so much!!

5 Answers


p0421- warm up catalytic converter, bank 1- efficiency below threshold, causes---catalytic converter, wiring, or H02S2 sensor


I know that's the error code but do you know with my other things that happened which is the most likely thing wrong and how Big a job it is to fix it? BTW it's been starting all day now and working fine so don't know what that means either Thank you so so much


could most likely be your wires, if it happens again while raining have it checked out.

Best Answer Mark helpful

Really appreciate!! You don't think it will happen before then? Or I need to get it checked out even if it's starting?


yes to check it out is always a good thing, but trust me to look for a fault that is not there is difficult.

Your Answer


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