Mazda3 P0421 Diagnostic Help Request

[treston]

Have a 2006 Mazda3 with a P0421 “Catalyst 1 efficiency below threshold bank 1.”

I bought the car with a blown engine, so it naturally had the CEL on the dash. After replacing the engine, this is the only CEL. Because I wasn’t there when the engine blew, I’m not sure when the code appeared, or under what circumstances.

On to A/F sensor readings…

Upstream sensor reads about -.05mA, jumping to +3.0 mA when the gas pedal is snapped.

Downstream sensor reads about .7 volts and fluctuates.

I also used my cheap HF infrared thermometer and read as close before the cat and as close after the cat as I could. I got about 100 degree F drop between upstream and downstream.

Anything else I can try? Seems like A/F sensor 2 is reading rich, but my OBDII live stream showed the A/F mix to be at about 1.003. Really not sure if I should rip out the cat/manifold and replace it, or replace sensors, or if there’s even something else causing this code.

Thanks for any help.

Topic

[Billy]

Is that a typo, or is the downstream side of your cat really reading 100F colder than the upstream side? If that’s the case, then your cat is dead as a doornail. Assuming the car was fully warmed up when you took that reading.

[treston]

Thank you for the reply!

Unfortunately, not a typo. Reading was taken after 15 minutes of driving. I’ll take another reading after work, just to be sure.

[Billy]

There are two situations in which a cat will not fire off (and this cat is clearly not firing off).
1. The cat is dead.
2. The mixture is so insanely lean that there is nothing for the cat to burn. In this case you would almost certainly have severe performance problems – stumbling, etc. You can double-check by looking at your fuel trims. If your long term fuel trims are maxed out, probably 25%, then something’s up. They should be within 10% of zero.

[treston]

Just checked the exhaust temp after driving it for ~15 minutes and revving it up to 3,000 rpm while measuring. Got 600f right before the cat, and 440f right after.

No performance issues, as far as I know, having bought it dead.

Just checked long and short term fuel trims. At 2,000rpm, long stays at -3.9% and short alternates between -0.8% and 0.8%.

[treston]

Also want to note that this cat has 200,000 miles on it.

[Billy]

Cool man, reckon you’ve found your answer.

[treston]

Thank you very much. I had a feeling that was the issue, but I needed someone’s expert opinion.

Any idea where the emissions tag is on the car? Need to know if it’s a California model or not. Looked under the hood and didn’t see anything. There is a blank spot literally on the underside of the hood on the passenger side that looks like something should be there, but it’s empty.

[Billy]

Sounds like maybe that’s where the tag was, but I bet you can look it up by VIN on some Mazda website, or at worst call a dealer.

[treston]

I can’t find whether this car is a Cali emissions or not. If it is a Cali, and I put a Fed cat on, will it throw a code?

[Billy]

Can a dealership really not tell you? If they can’t, I’ll bet a dozen Krispy Kremes that you can call Mazda customer support and they’ll tell you. If you live in a state that does emissions testing, you can contact those people and they can tell you for sure.

[treston]

Dealer was able to find it for me. New cat is on the way.

What kind of Krispy Kreme do you like? 🙂

[treston]

I got the new manifold in this week. Ended up replacing the O2 sensors as well because the threads were not looking great.

Took it to e-check today and it passed.

Thank you for all your help, relative!

[Billy]

Nice work! Thanks for posting the resolution.

[Jason Weremy]

So I am having a similar issue with my 2008 Mazda 3, 2.0 with 240,000 kms(150,000 miles) in the last couple of weeks. We had a big cold snap around Christmas time and it was around -30C, not adding windchills. I wasn’t able to start the car under it’s own power or using a booster pack. I had left it until Boxing Day to where it would turn over with a booster pack. I drive the car around around the block a few times to let it warm up. The engine light of the P0421 remained. Over those 2 days of above freezing temps the engine code had went away. The car started fine for those few days. Another cold snap came in -30 to -40’s just after Christmas and the code reappeared.

The car runs at idle of 800 RPM once the car is completely warm. Driving it at highway speed it’s just under 2000 RPM. The p0421 engine code also stated that it wasn’t coming up to temp(The CAT). To me what would make sense since that it’s been in the -30’s for weeks. I have a Bank 1, Sensor 1 O2 sensor that I am going to replace anyways. But I wonder, if I change the O2 Sensor and I reset the light and it comes back, do I just assume that the CAT is messed and needs to be changed? It is 14 years old after all. Is there a way to check the CAT without just having to replace it? Can you even get a used CAT?

Before you answer. Just so you know. The Bank 1, Sensor 2 was replaced back in the Fall of 2021 as well as the spark plugs in the Summer of 2021. Everything on the car is completely stock. I do have an cheap OBDll scanner, no multi meter and the car is completely stock. Everything is original, just the Air/fuel sensor, spark plugs are new.

Any help would be greatly appreciated everyone! Thanks.

Much love from weekender DIY Canadian.

• This reply was modified 3 years ago by Jason Weremy. Reason: Missed some letters

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