My car is having trouble starting and staying running. It is mainly on cold starts lately but is intermittent.
In the past, it used to start for just a second and die. After troubleshooting it, I found a faulty ECU. I replaced it with a used one and it started from then on.
Then while driving it down the road, it started cutting out. It started with small hiccups then got worse until it eventually died. I would sit there a while and it would eventually start after a lot of tries. It seemed to only do it when it was warm.
I even thought bad gas as it would do fine for a while.
Now after sitting all night. It starts and runs for 3-5 seconds and dies, or does not start at all. Eventually, it will start and then run fine, even stopping and restarting.
It used to show code 12 and 15 but only shows 15 now. I am trying to do the diagnoses but it has been hard to get it going long enough to get it warm.
I tried starting fluid and it seems to be a fire issue.
With such an intermittent problem I am not sure where to begin troubleshooting.

Thank you for any advice