Daughters car started shutting off and the CEL came on.

Had my friend put a scan tool on it and it showed a P1300 with cyl 3 misfire data.

So I replaced both coil packs.

The car ran better than ever.

She goes to visit her brother and as she’s getting ready to leave it shuts off sitting at idle.

Now its running rough and there is no CEL light.

Could this be a computer issue or something like a weak battery?

I did see the battery terminals could use cleaning, and i forgot to do it as i was in a hurry to get it done.

what do you guys think.

No CEL has me puzzled.

 

The CEL went off after the coil change?

If running rough could disconnect one cylinder ign wire at a time to determine if it now runs worse or the same.

Misfire could be something beside a coil. Injector, EGR stuck open, low fuel pressure (fuel pump) among others. The EGR should be off at idle and of open can kill the engine.

Did the MPG change as in increase with the code?

If the battery connections are electrically poor this could cause a host of issues. If you disconnect the batter make sure the radio doesn't have anti-theft system that will lock it if power is disconnected.