I have a 2000 Montero Sport. Got code PO421 and have sequentially replaced all 4 O2 sensors in addition to having a complete tuneup including a new timing belt. After replacing the 4th and final O2 sensor I thought all was well but 2 weeks later the same code reared its ugly head. Where do I look from here, could it be my cat? The car seems to run just fine.
p0421 is a catalytic converter efficiency code and since you have already replaced the 2 post cat O2's then it sounds like you have a cat that has failed. Which bank is it?
December, 16, 2006 AT 1:06 PM
PO421 is bank 1. How many cats are there on this thing two or three?
I wonder how long I have til it goes bad on me.
January, 25, 2007 AT 2:05 PM
What is the difference between this code and PO420?
PO421 says " warmup catalyst efficiency below threshold (bank 1)" and
PO420 says" catalyst system efficiency below threshold (bank 1)"?
February, 14, 2007 AT 10:39 AM
Just for everyones info. I ended up taking my Montero Sport to Meineke and was told I needed a new cat, the owner of Meineke told me a direct fit would be $900 plus but that he could find one for much less than that probably even half. Well after searching for 2 days he calls and tells me he can't get an aftermarket direct fit and I need to get it from the dealer for the original estimate of $900 plus.
I am starting to get very bitter at this thing, for one thing I bought a car with California emissions (who ever checks for that when they go out to buy a car in Pennsylvania). Well at 44 I'm still living and learning LOL