Thread: p0420
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Old 02-25-2011, 02:15 PM
rdl rdl is offline
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Location: Ontario, Canada
Join Date: Jun 2010
Posts: 1,767
Mein Auto: 530i 2003

Problem could be the cat, or might be a bad post (after) cat O2 sensor. A lot cheaper.
Before buying anything, try swapping the after cat O2 sensors. Get codes cleared and drive again through a few cycles as decribed below. If the fault follows the O2 sensor, i.e. P0430 Catalyst System Efficiency Below Threshold (Bank 2) appears, replace the sensor setting the DTC.
If the code disappears, you probably had a dirty connector that cleaned up when you unplugged / replugged during the swap.
If the code stays with bank 1 it's probably time time for a new cat.
Or find a really good diagnostic guy on the chance that the car has some other combination problems cheaper than a cat. It's a long shot but maybe. For instance, you mentioned a loose exhaust part. If it allows entry of even a tiny bit air before the post cat O2 sensor, it would then read high O2 level and set the fault.

EDIT: The ECU calculates P0420 or P0430 by comparing readings from the pre & post cat O2 sensors. It expects a better reading from the post cat sensor since the cat should be cleaning up the exhaust gas flow. From the difference, it calculates how well the cat is working.
It's unlikely your cat is clogged as the Autozone guy told you. A clogged or broken cat will restrict exhaust flow and you would notice degraded power, expecially at high RPM. In some cases, you might feel the car bog at highway speed & then regain some power if you take your foot off the gas pedal for a moment and then step on it again - only to repeat the cycle a few minutes later. Since you report the car has good performance, the Autozone diagnosis looks suspect.
However cats do loose effectiveness over time or due to poisioning by contaminants - which is the reason for the post cat O2 sensor.

The instance when your CEL came back after fueling is a likely a consquence of some DTC's being set only on consecutive repeat occurences. The sequence of events was most probably went way:
1 you cleared DTC
2 ran engine up to heat which activated O2 sensor operation (1st cycle)
3 ECU saw/calculated a P0420 and stored a pending code. No CEL set
4 you stopped and fueled car
5 ran engine up to heat again, ECU sees P0420 again, 2nd consecutive cycle
6 ECU set CEL

I can't think how the rush of air out the gas cap you describe could be related. It may though indicate that your evap recovery system is having problems.


Last edited by rdl; 02-25-2011 at 08:44 PM. Reason: Clarify reason for code
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