My mother-in-law's van, a '10 Caravan 3.8L, now with 220K, has had an ongoing P0420 for some time now. Replaced the upstream O2 sensor with a Bosch piece last fall, probably 30,000 miles ago?, but it did nothing for the code. Gas mileage was respectable (as much as 22), performance wasn't bad, so we just left it alone. The van doesn't have the best maintenance history - I'm sure it's been run low on oil most changes, no trans. service, FIL couldn't recall having put plugs in it since they got it at 20K. Basically only what it needs, when it needs it. I've been maintaining it about the last 2 years, keeping the OCI's between 6 and 10K with Maxlife, rotating tires, etc., but not doing any other real preventative maintenance to the drivetrain.
The last few weeks she's noted what she calls a stumble, first with cruise control on but not set, then this week regardless of cruise status - only at highway speed, after coasting off a hill and stepping back into the gas. I never witnessed it but that description coupled with the tranny's lack of service could mean the coast/lockup in the converter getting funky. So I got ahold of it to do the OC and see what I could find. Codes present were the usual P0420 and a new one, P1128. They needed it back in short order so I also did plugs, air filter, PCV valve, and cleaned the throttle body in one fell swoop. On the drive back (20 miles) it behaved perfectly. Poking around the net I've found some mention of P1128 being related to a dirty throttle body and faulty TPS feedback that affects the transmission shift behavior, which could confirm why what I did yesterday fixed it. Any further knowledge out there on the validity of this theory?
The last few weeks she's noted what she calls a stumble, first with cruise control on but not set, then this week regardless of cruise status - only at highway speed, after coasting off a hill and stepping back into the gas. I never witnessed it but that description coupled with the tranny's lack of service could mean the coast/lockup in the converter getting funky. So I got ahold of it to do the OC and see what I could find. Codes present were the usual P0420 and a new one, P1128. They needed it back in short order so I also did plugs, air filter, PCV valve, and cleaned the throttle body in one fell swoop. On the drive back (20 miles) it behaved perfectly. Poking around the net I've found some mention of P1128 being related to a dirty throttle body and faulty TPS feedback that affects the transmission shift behavior, which could confirm why what I did yesterday fixed it. Any further knowledge out there on the validity of this theory?
