Hey guys, I have a 99 dakota ext. cab with the 3.9L 4x4. Has 102K miles on it. Today i had the check engine light came on and its coming up the code p0420 which is faulty catalytic converter. well three months ago i had a new magnaflow cat put on and a new 02 sensor in the rear. so its not that. the sheet print out from autozone says it could be a vaccum leak or an oil leak, possibly or most likely from the piston rings. anyone else had this problem before or have any insight before i have my engine torn apart to just check the piston rings or search for the needle in the haystack trying to find the vaccum leak?
any help is appreciated.

Justin

 

if you aren't burning any oil or have coolant leaking into your exhaust, chances are you will need a new upstream (front) oxygen sensor. Since you put a new cat and rear O2, the front O2 is old and not varying the voltage to the computer fast enough for the computer to be happy with. Make sure also that you don't have any exhaust leaks. If you like to know more, visit your local emissions testing facility and get a readout of your exhaust gases. I'll explain it you in greater detail; I deal with emission issues all the time.

 

I live in Ohio so I just had my e check done in June when I had to renew tags and it passed but I don't know the exact readings. Everythings all digital these days. I know there's some exhaust leak somewhere along the down pipe on the passenger side because I can hear under acceleration when the windows are down echoing off freeway walls or trees. But I have yet to find the leak. How would an exhaust leak set that code off though? And no I don't have an oil leaking Into anywhere as far as I can see

 

if the exhaust leak is before the upstream oxygen sensor, it will send a signal to the computer that the engine is running lean and that it needs to add more fuel. If your exhaust leak is downstream past the sensors, it won't have any affect on them. Try pulling out one of your spark plugs and see what the condition of it is; if it's black & sooty, you know you have a rich problem.

Another thing that many people don't realize is that catalytic converters don't just go bad on their own, they're murdered; usually the result of too much fuel or engine mechanical problems (i.e. oil or coolant). So if you do have a bad catalytic converter, the root cause has to be fixed, if that is indeed the case.

 

My cat is a brand new magnaflow so I know that's not the

 

My cat is a brand new magnaflow so I know that's not the Problem. I'm going to my buddy's house that knows a bit more than me to take a look at it. I'm thinking about changing to the airlines 3923 plugs while I'm at it. He thinks it's the exhaust leak I have. Could very well be the upstream sensor also since that has never been replaced

 

My truck has the same 3.9 you have but i have 246.000 on mine,I replaced the o2 sensors twice in the last 6 years and the new ones last for about 4 or 5 months until they start throwing the p0420,i have honestly givin up reseting the computer and buying $100 worth of o2's again,the funny thing is that the trucks performance or fuel consumption does not change,it's just a light that annoys the sh*t out of me.If i leave it alone after about a week or two i will get in on a cold start up and it will not be on(check engine light) then it will come back on in about three or four more day's,i think it is a scam that the chrysler/dodge plant put in the computer to make you worry so you take it to the dealer so they can bend you over and throw a thousand dollar invoice at you,lol

 

when my originial o2 when out, it didnt set off the p0420 though. but if it is bad already then there has to be some kind of underlying problem somewhere thats doing that...right?