Hmmm...sorry if this shows up as duplicate post. I don't see my first attempt actually popping up.

Shorter version of the original: Code P0420 - If you look at an info page from YourMechanic on that code, you can see the stuff listed out that might actually cause it. At a minimum, at least you'll have a better idea what the shop might tell you is wrong when they check that specifically.

https://www.yourmechanic.com/article...-jamahl-walker

Hope you get a simple solution for this.

Hmmm...my response didn't come through, twice in a row. P0420...Misfire, oxygen sensor, or fuel injector. I was trying to pose an informational link to details about the code, but...shrug. EDIT: Okay, that worked. Is it against the rules to post links?

Sorry for the repeat post. I was trying to give you a useful link to information regarding that code, in case it proves useful. YourMechanic sometimes has some really useful stuff. If it turns out your car wasn't dying because of something caused by the code, at least you have some idea what else the shop might recommend when they look at it.

Trying again, without the link tag.

https://www.yourmechanic.com/article...-jamahl-walker

My wife was driving to work this morning and it shut off again. She pulled over, turned the key off, tried to start it and it wouldn't. On the second attempt she got it going. Could the immobilizer receiver possibly be going bad?

There is an immobilizer light that is key shaped and green. The light can indicate if there is something wrong with the system. Take a look in the owner's manual for how it indicates a problem. FYI, I rarely see this as a problem. If you have another working key, you can definitely see if the problem goes away.

We need some more detail on what happened when the car shut off. Did the dash act like it does when you first turn the key to the II position, with the bulb checks? When the car didn't restart, what was happening exactly?

Fair point on the immobilizer system. It comes on when the key is turned to ON and stays on for two seconds, which is normal according to the service information. When the key is OFF, it blinks five times, which I'm assuming is also normal.

As far as what happened when the car shut off, it was just like the key was turned off. Engine shut off, but no dash lights. It was in D4 but still rolling, almost as if I had put the trans. in neutral. Generally the car starts right back up after pulling to the side and turning the key completely off. One time when I parked in a lot and shut it off, I came back out and it took three tries to start it. It turned over with healthy cranking power, but nothing else abnormal. Hope that helps.