Code P0421 and terrible gas mileage

koffinb · 03-28-2023, 07:38 PM

I have a 2006 330i with a little over 200k miles on it. Car runs great, but a few weeks ago I had my CEL come on and scanned it with a generic OBD scanner. Came back as 'P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)'. I have also been getting really bad MPGs lately as well.

Any idea on what could be going on? I was thinking may O2 sensor? Anyone else have a similar experience with this code?

Serf27 · 03-30-2023, 10:48 PM

The o2 sensor is cheaper than a new cat converter.
I can’t find which sensor relates to P0421, but my guess is the rear sensor is bad.

You can diagnose it with a better scan tool/voltmeter.

03-28-2023, 07:38 PM	#1
koffinb Enlisted Member 21 Rep 36 Posts Drives: 06 325i sport package Join Date: Mar 2012 Location: United States iTrader: (0)	Code P0421 and terrible gas mileage I have a 2006 330i with a little over 200k miles on it. Car runs great, but a few weeks ago I had my CEL come on and scanned it with a generic OBD scanner. Came back as 'P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)'. I have also been getting really bad MPGs lately as well. Any idea on what could be going on? I was thinking may O2 sensor? Anyone else have a similar experience with this code?
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03-30-2023, 10:48 PM	#2
Serf27 Brigadier General 2851 Rep 4,322 Posts Drives: 335i Join Date: Apr 2018 Location: Sitting in a BMW. iTrader: (-1)	The o2 sensor is cheaper than a new cat converter. I can’t find which sensor relates to P0421, but my guess is the rear sensor is bad. You can diagnose it with a better scan tool/voltmeter.
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