A short while ago I replaced the front O2 sensor after a CEL showed it was bad. Now (a couple gas tanks later) I'm getting P0421, indicating the front (pre-heat) cat has gone bad. Probably the failing O2 affected the cat, which has 150K miles. But I'm wondering if this code could be triggered by a failing 2nd O2 sensor, the one behind the pre-cat.
Does the P0421 usually just mean replace the cat?
thanks,
Remember that the cat is only warrenited for 80,000, so, from the mfrs standpoint, it should fail at about 100,000 so that you buy a new one, or trade/sell the car.
In other words, the cat may actually be bad. Remember that aftermarket midpipes with cat are less expensive than the factory part. (I'm not saying that they are CARB approved)
On my A6 Avant, I have been getting these same codes since at least 80K in
2004. P0421 (Pri Cat Bank 1), P0431 (Pri Cat Bank 2), and P1423 (secondary air flow too low bank 1) always show up 1 or 2 days after being cleared.I can guarandamtee you these cats did not go bad at 80K.
How to know? I always knew those emissions tests in Denver would be good for something. In 2004 at 66K, my hydrocarbons were 0.07 gm/mi and NOx was 0.2 gm/mi. In 2006, at 136K HC were 0.13 gm/mi an NOx 0.3 gm/mi. Pass/fail cutpoints are 1.2 for HC and 3.0 for NOx. This car is pretty darn clean from an emissions standpoint.
If the cat was shot, HC would be much higher, trust me, people I know who have had bad cats have failed the emissions test hands down. Even if my cat is bad, why would I replace it if my car runs that clean? That makes no sense.
I need another emissions test this year (177K) so will do that before taking to my mechanic this week. I will try to have him do a pre- post- cat temperature reading.
The fuel economy on this car has always ranged from 20-22 mpg, so I don't believe it is an O2 sensor. I believe it is a secondary air leak issue that is of very minor concern (except for that damn CEL/MIL :x ). Hope to have a diagnostic on that this week.
Will let you know what transpires.....
If you have a california style cat system-- the front cat does have a history of high failure rates.
That aside, the 99 manual lists these possible causes for the P0421 code in the following order. Warm Up Cat exhaust system leak Front O2 sensor Rear O2 Sensor Mass air flow sensor Throttle position sensor Vehicle Speed sensor. The process of elimination in the manual involves use of Mazda dealer specific equipment. The presence or absence of other codes may help eliminate some of the posibilities, as should normal indications of the speedometer, etc. Not a few have changed out the calif spec exhaust system to federal spec, eliminating the warmup cat.
Is there any information on changing a California car to Federal spec on the Internet, or does anyone sell a kit? I have a CA car with a bad warm-up cat (due to a couple of misfires caused by a bad coil) and I'm in AZ, so I'd rather simplify the system rather than install a part with a history of going bad on people. Or, does anyone know of an aftermarket warm-up cat? Thanks for any info you can provide, Mark
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