Error Code: 0421 - What's the problem??

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kymp3

Member
I recently bought a 03 Mazdaspeed with 10,000 original miles. It currently has a 2 stage boost controller, Tein SS coilovers, and a Greddy SP exhaust with downpipe installed. The check engine light came on and the code being shown is P0421 - Warm up Catalyst Efficiency Below Threshold (Bank 1). Would anyone happen to know what this code means, and the possible fix?(huh)
Thanks for helping out the new Mazdaspeed Owner.
 
it means your first cat is not working the way it is supposed to. if there is no cat, thats the problem. the first and second o2 sensor need a cat between them. if there isn't one, the code 0421 will get stored.
 
According to my factory service manual:

P0421 indicates Bank 1 catalyst system efficiency is below acceptable threshold.

POSSIBLE CAUSE

- Use of leaded fuel
- Oil contamination
- Cylinder misfiring
- Damaged Ho2 sensor
- Malfunctioning ECT/CHT sensor
- Downstream Ho2 sensor improperly connected
- Fuel Pressure too high
- Damaged exhaust system pipe
- Damaged exhaust manifold
- Damaged catalytic converter
- Retarded spark timing.
 
Keep in mind:

- Internal deterioration of a catalytic converter is usually caused by abnormal engine operation upstream of the catalyst. Events that can produce higher than normal exhaust gas temperature (EGT) are particuarly suspect, such as misfiriring.

- Incorrect Ho2S signal input (e.g. rich/lean signal when engine is operating under rich/lean conditions) can cause an abnormal temperature increase in the catalyst.

-ECT/CHT sensor DTCs can indicate that thermostat is not operating correctly or that engine coolant level is not filled to specification, producing above normal operating temperatures.
 
There isn't a cat between the downpipe and the rest of the exhaust. I'm assuming that is probably the issue. It's just kind of weird how I drove it a couple of thousand miles and there was no check engine light display. Now all of a sudden, it's there. Thanks for the quick responses guys, huge help.
Do you think replacing the O2 sensor would make the light go away??
 
nope. you will either need to install a cat, or do some sort of cel cheater on the second o2 sensor.
 
it can't hurt to try replacing it but prolly the problem is that there were only 10k miles on the car. How many miles are on the car now? Wondering if it was driven only a few miles a day to and from work and the car never really got hot enough to cause the car to not be in 'run rich mode' during initial startup causing issues with the o2 sensor or worse, the cat?
 
Why do you think it took a couple of thousand miles for the light to eventually show up? It's been without a light the whole time I have had the car. It still runs fine, but the light is so annoying to look at all the time.
 
Bought the car with 9900 miles two weeks ago. It is absolutely mint. The guy I bought it from kept it garaged in the winter and only drove it during the warm months. New car smell and everything. But now the engine light. Kind of ironic, I went down a road that was horrible and bumped the exhaust, and right after, the light came on. No damage to the exhaust, but the CEL has been on ever since.
 
kymp3 said:
Bought the car with 9900 miles two weeks ago. It is absolutely mint. The guy I bought it from kept it garaged in the winter and only drove it during the warm months. New car smell and everything. But now the engine light. Kind of ironic, I went down a road that was horrible and bumped the exhaust, and right after, the light came on. No damage to the exhaust, but the CEL has been on ever since.
Click to expand...

check if the catalytic converter isn't damaged, and check the Ho2S wiring.
 
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