OK, I got an error code P0420 Catalyst System Efficiency Below Threshold (Bank 1), what do I do now? I got the error code only when the weather temps were 10 or more degrees below zero. I don't want to change the cat converter if it is something simple like replacing and oxygen sensor or such. Any suggestions?

 

Try cleaning the sensor first. Works most the time for any sensor, if it doesn't I would probably replace it.

 

Now I'm puzzled, replaced the sensor #1 before the cat and still getting the error code. Sensor #2 was replaced 6 months ago. Is it the gas I'm using that is causing the error becuase it is inconsistent when the check engine light goes on. One tank full and it will be fine, the next get an error code. My MPG's are fine and exhaust does not smell

 

the 420 codes comes from the rear o2 sensor. the cat is basicaly a oven when there are colder temps out the cat is losing its heat and may trip a 420 code.(reason why they installed heat shieldson cats.if u are missing the heat shield this can be your whole problem in cold weather)a consistant 420 code is a sign of a catalyst deteriation meaning you are on your way to replacing the cat. if u have acces to a laser thermometer take a temp reading in front,middle,and rear of cat the temps should be around 225 degress (giveor take 20-25degrees)when fully warmed at a idle the middle of the cat will be the hottest. when the d is cold try hitting the cat with ur hand and listen for any ratteling (dont confuse this noise with a lose heat shield)if you hear any ratteling your in for a cat. option number 3 go buy spark plug antifoulers drill out the centers remove rear o2 install antifouler reinstall o2 into antifouler this may require 2-4 antifoulers to keep light off but this is only a temporary solution and if the catalyst is damaged the deteriation of the cat will lead to failure. worse case with the failure the broken catalyst will become lodged in the exhaust system rendering poor performance.

 

If you use lower-quality replacement parts for things like o2 sensors, they can fail quite quickly. I've seen that many times. Did you use OEM or high-quality o2 sensors? The rear sensor could be bad again or it could be your converter. I found this link with some tips - http://www.obd-codes.com/p0420

 

Have you completed a back pressure check yet?

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