ISX12 500EV bad de-mandated club

[thewondershowzen]

Hello and thanks for letting me join. I am working on a firetruck that has an ISX12 in it that has been de-mandated and now has more trouble than before.

When this truck came in our shop to troubleshoot a CEL, it only had codes for a bad def head and the department decided they wanted everything removed since it wasn't the first time it had happened.

(We do not remove anything in our shop, but have sent people to this local business before with great luck) Now the truck is not correct and I am obligated to get them taken care of since I recommended this shop that has said they have done everything they can and that the truck needs to go to the stealership to diagnose a bad wiring harness. (rolls eyes)

After a local sh*p de-mandated it, (gutted both cans, and plugged the EGR) they could not get it to make enough power to pull 3rd gear for a week. (red flag, I know but) After them reflashing, reflashing, and reflashing, the truck seemed to run with decent power. But...
It will backfire through the exhaust about 30 seconds after being turned off almost every time the truck is drove. They took the truck back to troubleshoot and said that everything checks out on their end.

It has excessive white smoke on startup and a haze when running. The jake brake does not function, and the check engine light will come on with a soft code set in the transmission for engine overspeed when the RPM's exceed 1000-2000 RPM (not always consistent at what rpm, but will never go over 2000 without setting). P0726
The check engine light goes off after every ignition cycle, but comes on as soon as it is driven.

I have since checked every plug on the engine and transmission harness for bad or corroded connections and changed the engine speed sensor just because, all with no change.

I really feel they got a bad tune and would love a second opinion.