Ok, my wife was starting her car yesterday and she said the car was rocking when she started it, and then the service engine soon light came on. It did it again this morning, she called me out there to look at it and when I got in and started it, it only did it for a second and it didn't seem like anything to me. She said before I got out there, she started it and it started rocking for a few minutes. I didn't see it so I have no clue. Has anyone seen this before? I just want an idea of where to look before I start tearing things apart. The car is an 02 OZ Rally and has about 105k miles on it. Any ideas?

 

Auto? Manual?
Is it really cold when it happens?
Is the engine light still on?
Where you able to get the code out the ecu?

My guess...
The car is manual, it's fairly cold in the morning, and the car is parked in gear.
When she starts the motor up, the engine goes up to 2000rpm and stays there for a while to warm up.
While it's idling at 2000rpm, she takes her foot of the clutch and the car starts rocking back and forth (remember I assumed the car was in gear?)
Since the parking brake is engaged, the car doesn't go forward nor does it go backward. It just rocks back and forth.
She turns it off, you get there, start the engine, put the trans in neutral since you know it shouldn't be in 1st if you let the clutch go. Hence why you haven't seen it happen your-self.

Now you're gone tell me it's impossible, the motor would just shut off in such a situation.

Not.
I've had that happen and it almost costed me a fender.
I got my car parked by a parking attendant once, and the guy left it in gear. I never leave my car in gear; never.
I got out of work, hoped in the car, started the engine and released the clutch pedal. The ebrake was fully engaged, but with the engine reving at 2000rpm to warm up, it managed to pull me forward at a reasonable speed. I barely missed the GrandAm that was parked in front of me(3 meters away).

That's my assumption. I could be wrong

 

Its an automatic, she went to kragen today and got the free diagnostic. He told her to try the spark plugs first, and then gave her the list of possible causes. The catalytic converter defective; engine misfire or running condition; large vacuum leak; engine oil leakage into exhaust-valve guide seals, piston rings. I believe the code was P0421. It wasn't cold outside, we've been having a pretty bad heat wave here in San Diego. The service engine soon light is still on. The spark plugs should be replaced tomorrow and we'll see how that goes.

I let someone drive my Evo one time and I didn't see he left it in gear when he turned the car off, and my car lurched forward pretty hard when I started it later that day. I never leave my car in gear either.

 

First cat converter is shot, or you didn't go for the recall. Mitsu issued a recall and they need to flash the ECU.

 

Yeah I found out about the recall after I posted, I did a search on the code and it came up with that. We have an appt. with Mitsu tomorrow, thankfully it's still covered so they're going to do it.