Warm up catalyst below threshold - Kia Forum
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#1 (permalink) Old 03-21-2008, 07:57 PM
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Question Warm up catalyst below threshold

I was driving down the highway earlier and on popped the check engine light. I took the car to advance auto for them to put it on the computer and it came back as " p0421 Warm up catalyst below threshold " got any ideas on what the heck that means??
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#2 (permalink) Old 03-24-2008, 11:25 AM
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this is probably relating to the downstream O2 sensor behind the cat. It could also be pre-cat. But almost definetly an O2 sensor as those are the only sensors related to that system and all modern O2 sensors are of the 'preheated' nature which means that they use heated element within the sensor which measures the exhaust temp and mixture to send to the ecm to properly meter air and fuel to the cylinder.

i also found this for a Mitsibishi recall problem, shows could be a problem with the cat internally which might explain the temp code, downstream sensor may not be reaching temp because of plugged or otherwise damaged cat.

'The MIL may illuminate due to the PCM's determination that the catalytic converter has deteriorated. Code P0421 is found. Separation of the catalyst layer and melting damage to the substrate layer has occurred in investigation of the damaged vehicles. This could be due to unburned fuel igniting close to the converter. Remedy: Reprogram the Engine Control Unit with ROM upgrade. A second letter sent to owners of vehicles built prior to January 2002 to insure the throttle position sensor is properly initialized.'
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#3 (permalink) Old 03-25-2008, 05:26 PM
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Sweet Reply! ^^^^^^^^

A couple of easy ways to tell if your Cat is bad is simple.

1) Smell, if you smell the exhaust coming out of the pipe and you get a burnt metal, burnt egg or even a burnt oil smell, it could be clogged up.

2) Sound, have someone drive your car on an open highway, or you do it, but have someone following in another car and have them place their car right where the exhaust is on your car. Then FLOOR the gas for a couple of seconds and see if there is a HISSING sound coming out of the tailpipe.

If you smell or hear any of the above, then your cat is definately bad.

If you DON'T smell or hear any of the above, then look at your 02 Sensors. Your car is old enough to need them.

Also, try putting a can of BERRYMAN B-12 Carburator/Injector Cleaner in your gas tank every oil change. It will clean from the gas tank to the tail pipe.

I've been using it for over 25 years.

When we bought our 04 Optima, it still had the 2006 Emissions test info. I had to get it inspected in 07 and before I took it down to get it inspected, I put (2) 16 oz cans of that in with a full tank of gas, and once I was down to about 1/4, I fill the tank and took in to get inspected.

Our emissions were almost non existant! They guy said our car posted #'s that nearly equalled that of hybrids!

I checked our NO2 and Hydrocarbons and the numbers were way below the 2006 inspection.

So try it. But remember, put 2 in the tank and fill w/ super unleaded and run the heck out of it. If the cat can be saved, this would do it.

Need Help? Let me know. I might be able to answer your question.
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#4 (permalink) Old 04-20-2009, 10:34 PM
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I have a similar code to this it is P0431. "Warm up catalyst below threshold (bank 2)" Do I only need to follow the same instructions as above. I like the idea of just adding cleaner to the fuel, nice and cheap. Where is the bank 2 sensor?
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