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Discussion Starter #1
So I have had my CEL on for awhile now. The other day my wife went to take the car out and the CEL started blinking and it was missing pretty bad. When I got back in town I started it up again about a week later and the same thing, I let it run for a min and it smoothed out...CEL stopped blinking and just went back to solid. I took it to O'Reilly's (a few blocks away) and checked the codes.

P0300 - Rand. Mult. Cylinder Misfire
P0302 - Cylinder 2 missfire
P0303 - Cylinder 3
P0306 - Cylinder 6
P0118 - Engine Coolant Temp Circuit High Input
P0421 - Warm up Catalyst efficiency below threshold bank 1

Now I know they are not all related but I'm sure some are. I've been doing some reading here and some say P0300-306 could be nothing if it works fine after a bit, or could be a few other things. Also, I have been having problems with heating, heat is fine when I'm going, no heat when not under load. I've read this could be from air in the cooling system, or just a build up of junk. I plan on doing the write up on flushing the heater core if it gets warm enough around here. Anyway, I just wanted to give some background relating to the problems I have had lately to try and narrow down the possible issues.

As for the P0421, I have noticed a drop off in gas mileage since the CEL has come on for that. Could be O2 sensors or maybe even cat?

Thanks for the help all...

2002 GLX V6 4motion - 133K
 

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Discussion Starter #2
Any ideas on where I should start? I just don't want to spend money on stuff that may not be needed, and if some of these codes/problems may be related, it will give me a good starting point.

Thanks
 

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P0421 - Warm up Catalyst efficiency below threshold bank 1

This is for your cat, check it out. If you have the atq engine, there is a TSB for that code and P0431 which more or less means an ecu update would make that go away if the cat isn't bad. Mileage does go down with the code.
Something you can look into...
 

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Discussion Starter #5
i guess it could be a bad thermostat, however, I had my TB done about a year and a half ago and had it replaced along with the water pump. Prolly should have mentioned that.
 

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The heat problem could be a bad thermostat.
So could the P0421, in my not-so-popular opinion. When my thermostat was stuck open, I'd get P0421 whenever it was cold out and the engine couldn't fully warm. Come spring (or a new thermostat) and the code went away.
 
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