2014 GMC Sierra 5.3L 180,000 miles approx. but scan tool shows 255,000 miles.
DTC'* current.
P0128-00 engine coolant temp below thermostat regulating temp.
P0513-00 Immobilizer key incorrect.
P06DD-00 engine oil pressure control solenoid valve stuck off.
B1517-03 battery voltage-low voltage.
B1325-03 control module power circuit-low voltage. HVAC
U1509-00 lost communication with device on LIN bus. HVAC
U0020-00 low speed can bus.
original complaint was no heat and fans running all the time.
I have changed the thermostat and temp sensor and it didn't fix anything. battery is only 6 months old. It starts and runs good. Coolant is full at the overflow tank.
after driving 15 minutes temp gage reads 160. also shows 160 on scan tool. battery reads 15.1v HVAC module seems to perform all functions correctly, however, when I feel the upper rad hose it is luke warm barely. one heater core hose is luke warm and the other is cold.
I can clear all the codes and fans shut off but everything else stays the same. still no heat.
Also one last thing, occasionally he says he gets a crank, no start. lets it sit 10 min and then it starts.
let me know if you have any ideas. I think he has an electrical problem but that doesn't explain the no heat issue. this is my brother-in-laws truck. he hasn't had time to let me really dive into troubleshooting but now that it'* 15 degrees outside he has made time. LOL. it'* been doing this since last summer.
Thanks

just an update. after clearing the codes, he said the gage went up to 185 and he had full hot air in the cabin.

So we are good? Maybe an air pocket?

No unfortunately. I've cleared the codes twice before and after about a week it will start doing the same thing. I'm going to check grounds to engine and chassis tomorrow. and connectors to computers because they are under the hood and might have got wet. Other than that, I'm stumped.

How many miles on this vehicle?

How many miles has BIL owned it?

Has it been through any trauma in the last year or two like accidents, abuse, fire, flood, alien abduction?

Do you have any wiring diagrams for it?

about 180,000 miles on it.
He bought it new in 2014 from a GMC Dealer.
never been in an accident or anything bad like that. Never had any problems with it until now, it'* been very reliable. But he says it uses a lot of oil and it smokes a little when he cold starts it.
I don't have any wiring diagrams on it or service info.
I didn't get to work on it today but will tomorrow for sure.
He may be an Alien......

LOL okay cool. Thanks for the info! Looking forward to what you find!

okay here'* where I'm at. 3 1/2 hrs today working on it and looking at stuff.
very dirty engine bay, including oil on and around the PCM. Took the PCM out cleaned the pins on it with brake clean and did the same with the connectors. they looked ok. 2 other large connectors near fuse box, did that to them too.
Took the fuse box out and cleaned all that too. Blew everything out with my compressor. Don't think any of these are the cause of the problem. checked grounds on engine, frame etc. most looked good, but took off and cleaned anyway. checked battery terminals. neg seemed a little loose. so I tightened it. Could be a cause.
I did find a connector on front of the engine by pulleys that was absolutely soaked in oil. I tried to get it apart but could not reach it without taking a bunch of stuff off. I think it may be for crank, cam or oil pressure sensors. I sprayed the hell out of it with brake clean and will just have to see. GM connectors seal pretty damn good.
It looks like the front main seal is leaking oil and both valve covers so that is getting oil on stuff. I'll tell him to get it fixed.
Everything looked as it should so we will just have to wait and see if the problem comes back.
I saw videos about grounds inside by A pillars that cause problems for a lot of people so I'm going to look at those tomorrow.
I will keep you posted. Sorry if I'm rambling.

Thanks for the update!

I've never seen an LS leak that much. I wonder if someone has been in it.