I had the service engine soon - p042 - catalytic converter error and fortunately for me it happened with 79500 miles. The dealer had it for TWO WEEKS because it was the side that they had to drop the engine to repair.

I just got the car back yesterday, and now it is missing while in first gear. It doesn't seem to do it when I first shift to drive, it's after it's been in drive and I slow to a stop, and then try to accelerate. It takes a second when the RPMs rev before the car will take off. It's even squealed the tires once when I was trying to take off. It was not happening before I took the car in.

I have to call the dealer this morning to get it back in, but I have the feeling their going to say it has nothing to do with the repair they did, but I obviously have my doubts. Anyone with knowledge on the subject care to chime in?

What year is your vehicle? My guess is that the work was done under warranty and you didn't pay out of pocket, correct?

I can think of 2 scenarios:
It could be electronic, if the battery was disconnected all along and you could at minimum try to reset your computers using the procedure in the FAQ/HowTos thread.
It is also likely you are low on transmission fluid since the system has to be opened while removing the engine and some fluid could be lost; if they really "dropped the engine".

Either way, it doesn't hurt to ask, but if the car drove fine going there, and immediately after their extensive work it doesn't, well...it is more than a coincidence to me.

I'd like to ask a favor - could you ask the service group whether they still have the old unit around and if you could take a look at it?

Thanks for your input. It's a 2012. They're coming to pick the car back up tomorrow morning. What do you want me to look for if they still have the parts?

Alright, great that they'll follow up. Looking forward to their update.

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Regarding the old part, yours is not the first case of a V8 getting the P0420 code. A few of us are trying to figure out why the V8 and why the left one only.

My guess is that there is a crack somewhere on the collectors, but a disintegration of the catalyst material is not unlikely. I'd look down the tube and maybe shake it to see if something moves (nothing should).

If you paid for the replacement or they somehow allow you to take it, I'd love to know what's wrong with it and I'm willing to pay to get it.

Yeah it was under warranty because it was under the 80k. I'll see what I can find out.

Thank you. I understand that no promises are being made, but any effort helps.

... And of course, that the driveability issue gets sorted out as well. The only other person who reported having the P0420 issue AND actually replacing the manifold didn't claim any driveability issues afterwards, but that's only a 50% success rate based on forum data.
Sharing of your experience and eventual solution is good for posterity.

Correction: I remember 2 other people have had the work done at the dealership - biggie and SnowNYC1. Better numbers, but still only 67%

Apparently it was throwing a code for deficiency, so they said it was most likely clogged.

EdBwoy wrote: It is also likely you are low on transmission fluid since the system has to be opened while removing the engine and some fluid could be lost; if they really "dropped the engine".

You were dead on. Dealer says it was low on fluid. They just dropped it off this morning so I haven't had a chance to drive it, but they assure me it's good to go.

Boom! We could count it as your 80k mile transmission service.

Thanks for the info on the catalyst too.