98 grand am se 3.1 code po420

[weissmertz]

ive had this car for about 6 months the "service engine soon light" came on . the code is po420. ive still been driving it .it runs fine "BUT". i recently passed a car at about 40 mph. im sure it downshifted to 1 really impressed me on the get up out of a 3.1 "BUT WHEN I STOPPED AT A GAS STATION AND RESTARTED THE CAR IT WENT INTO GEAR SLOWW!!. i figured i drug up a bunch of sludge so i had the filter change. 138.000 miles just got worse. now it shifts into gear fine until it warms up then it goes in gear slow. if you rev it up before it usually acts normal. also if you come up to a stop fast and imediately give it gas it hesitates and clunks into gear. any info would be greatly appreciated

[Knifeblade]

PO 420 code is~~~~

TWC system low efficiency Bank 1

Sadly, that makes no sense to me, what TWC means? Kinda sounds like a transmission monitor of some sort from the symptoms U describe.

Hopefully, someone who knows more will chime in with some help, sorry I can't help more than this.

Tam

[justin0717]

TWC is three-way catalytic converter. code p0420 is set when the pcm thinks the cat is bad. what happens is there are two oxygen sensors (o2), one before and one after the cat, and the pcm compares these two reading to see if the cat is doing its job (reducing HC, CO, NOx).

If you are gunna try and see if the cat is truly bad or not, it take a look and make sure there arent any exhaust leaks around the 02 sensor behing the cat (or anywhere, could possibly set the light). Also there must be something that cuased the cat to go bad, which needs to be found out, something such as; Engine misfire, High engine oil or coolant consumption, Retarded spark timing, Weak spark, if the cat is bad, youll have to see which of these caused it to go bad. Things such as and old battery, weak coil, fouled plugs, ect. hope that helps

[weissmertz]

thanks for the info any ideas about the tranny symptoms

[Knifeblade]

Thanks, Justin, that helped me, YAY. yeah, '98 cat. shouldn't be going bad by itself, I agree something else than the cat. itself is causing the code.

Tam

[Flatrater]

justin0717 has it mostly right. But the O2 sensor in the rear of the cat is what the PCM uses to determine if the cat is functioning correctly. The PCM is looking for this O2 sensor to be steady and not see much of a change. WHen the PCM sees a constant change oin the O2 sensor readings it will turn on the SES light and store the P0420 code.


Now on to your trans problem which I believe is also tied to your cat problem. If the cat melts it will restrict the exhaust causing a loss of power making it feel like you have a trans problem.

If you want or can get acess to your exhaust pipe coming off the exhaust manifold, remove the 2 bolts holding the pipe on to the manifold and take the car for a ride and see how it shifts. I will bet that the trans will shift fine.

And as justin0717 stated cats don't go bad on their own most of the time, something usually makes them go bad.

[weissmertz]

well im positive the the trans has a problem but not till it gets hot then it will not go into gear till you rev it up a bit also if you stop real fast it will jerk or slip into gear

[Forkliftguy]

Here 's something to think about: An O2 sensor doesn't begin to operate until it reaches close to 600deg F, until that happens, well after the engine itself has warmed up, the emissions system runs in a closed loop mode, ie it doesn't pay attention to the cat. So in theory you should have decent performance from your car in that time period. Once it's ready and starts monitoring the "bad" cat it will start altering fuel maps which will give you poor performance (less power) in order to maintain safe emission levels which would explain the slow shifts from the tranny due to lack of power.

Try Flat's suggestion of unhooking the cat and see if there is any change in the tranny.

HTH
Marc

[weissmertz]

wel i had the cat convert change same symptoms w/ the tranny