HOW TO: Fix SES Error Code P0421 for about $5 bucks - Club3G Forum : Mitsubishi Eclipse 3G Forums
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Old 01/26/2006, 11:40 AM   #1 (permalink)

Join Date: Sep 2005
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HOW TO: Fix SES Error Code P0421 for about $5 bucks

This fix is pretty hard to find. I've had people give it to me in a link to other forums before. I figure since I just got done doing it, i'd post it here to help others.

One big point that I discovered during this research. DO NOT EVER GO TO DEALER! They love to fuck you in the wallet. Go to Advanced Auto Parts, PepBoys, Autozone, O'reilly's, etc. and they'll do a FREE code check and give you a print out. If you indead have P0421 this can help. Dealers want $50ish to do that!

This fix will basically get rid of a SES due to error code P0421 (Warm Up Catalyst Below Threshold (Bank 1)) caused by either a test pipe/down pipe (No Cat) or a High Flow Cat. When this light comes on from your install of a new exhaust/header/etc, be aware that this has no effect on the performance of your car. its just a stupid reminder that the o2 sensor is finding a problem involving something to do with the stock catalytic converter. Yes, by the way, an o2 Sim would do the trick too, but why spend upwards of $50 and have to deal with the cost/install/and problems associated with that kind of fix when you can just do this for about $5 bucks?

- You might have space for only 1 spark plug anti-fouler, sometimes using 1 will work, sometimes not.
- Having an electric drill helps out tremendously, those battery operated ones dont have that much power; they will still work but will be a pain
- This may still not solve your problem of the light; a small amount of people that try this 'quick fix' still cant get the light to go off, this could be due to having multiple error codes. It SHOULD fix P0421.

-purchase 1 package of 2 spark plug anti foulers at your local auto parts store
-18 mm size
-made by HELP! (Part #42002)

-disconnect the negative terminal on your battery. this will reset your ecu
-take one of the anti foulers and drill out its center using a 1/2 bit, this will increase its hole size and allow your stock o2 sensor to fit. If you can only fit one under there, use this one.

-screw the drilled spark plug anti-fouler on top of the original
-be sure to use some kind of anti-seize on the treads. in case you need to remove your fix with ease

-jack the car up or use a lift
-unscrew your stock o2 sensor and place the foulers in
-use anti seize on the threads
-clean your o2 (by wiping it off with a paper towel DO NOT GET IT WET!)

-reinstall your o2 sensor
-use anti seize on the threads

-reconnect your battery (Resets your ECU)
-for only about $5, and half an hour, you have solved code P0421 (Mitsubishi would have charged you an arm, a leg, and your 1st born child to do this)
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