Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1

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Sat 1 Oct 2022 16:24
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Can anyone help with their experience of a P0421 fault code, catalyst below threshold bank 1, the car seems to drive ok, maybe a bit jerky from time to time other than that seems fine.
I’ve checked for exhaust leaks, bad smells and the economy seems normal, I’ve viewed the live data for the sensors and seem normal, one sensor quite steady, the other fluctuates, it may seem the car feels underpowered at times but nothing dramatic, I’ve reset the code and everything is fine for at least a couple of weeks, sometimes 1000 miles or more, the only time it seems to bring the fault code back is after numerous short journeys.
Is this likely to be the catalyst failing/blocking or should I be looking at other areas too? The Mazda 3 has two cats, one in the exhaust manifold and one under the car, as far as I can see there are two sensors, pre-cat in manifold and post cat (2nd cat), I’ve yet to check the temperatures either side of the cats.
Any advice and experiences greatly received.
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Sat 1 Oct 2022 16:52
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - Adampr

It's probably the cat (in the exhaust manifold) failing. You could try replacing the oxygen sensors either side first, as that's the second most likely and cheaper.

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Sat 1 Oct 2022 17:14
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Thanks, thinking I may change sensors as the cheaper option, dread to think what the exhaust cat will cost though, it also looks quite a job to remove too.
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Sat 1 Oct 2022 17:35
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
I doubt the sensors are in trouble. The code p0421 is a typical code when the cat is not efficient. You could try a lambda sensor spacer in the downstream one . This can trick the engine control system and prevent the mil coming on .

Edited by elekie&a/c doctor on 01/10/2022 at 17:41

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Sat 1 Oct 2022 20:35
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Thanks for the spacer advice, I’d never even heard of that.
How would this affect emissions for the mot test though?
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Sat 1 Oct 2022 21:03
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
Using a spacer puts the sensor out of direct flow of the exhaust gas , which can slow down its sensitivity. Very often the p0421 fault code does not affect mot emission test . However, engine fault light on is a failure.
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Sat 1 Oct 2022 21:15
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Thank you, definitely worth trying then??
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Sat 1 Oct 2022 21:29
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
I would have a go . About £10 on flea-bay .
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Sun 2 Oct 2022 13:30
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - Engineer Andy

I'm wondering, in addition to the other things suggested, is it worth buying some CAT cleaner (dosed into the fuel tank like injector cleaner is), in case one of the CATs is clogged, possibly through it being used predominantly for short trips from cold for a reasonable period of its life? They only cost about £10 - £20 for a bottle.

Never needed to try it myself, but they seem to get good reviews from people who've suffered from CAT issues.

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Mon 3 Oct 2022 13:17
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - Gibbo_Wirral

If you want a cheap aftermarket cat, try CATS2U.

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Mon 3 Oct 2022 17:41
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Do non genuine cats last as long as aftermarket though? Probably better price wise but not sure about longevity, I’ve not asked Mazda for a price yet, but will compare
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Mon 3 Oct 2022 17:55
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
I would not be surprised if a genuine Mazda cat is well over £1k . Aftermarket will probably be half that at least .
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Mon 3 Oct 2022 15:38
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Thanks, I’m going to give that a try too, just a bit bothered if one of the cats is blocked etc what damage it could do to the engine if I don’t get it addressed.
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Mon 3 Oct 2022 15:53
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
Are you sure it’s got 2 cats ? If it does , it will have 4 lambda sensors .
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Mon 3 Oct 2022 17:16
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - John Mcfarland
Definitely has two cats, one in exhaust manifold and one under car, with only pre cat sensor in manifold and post cat sensor after second cat, I couldn’t believe it at first either.
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Mon 3 Oct 2022 17:19
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - elekie&a/c doctor
Fair point . Seems a strange set up . But I guess it works.
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Tue 4 Oct 2022 09:52
Mazda 3 , 2.0 sel nav - petrol - P0421 - catalyst system below threshold bank 1 - kiss (keep it simple)

Would a good long fast run help to clean it up?

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