Bono

The harness swap keeps all of the factory plugs for parking sensors, for lights, etc. The $400 harness is just an adapter with resistors to make the different plugs match. I’m trying to sort out where those need to be added.

Bump, low beams won’t stay on when engine is running.

I did a full front end swap from a 2018 Sierra Denali onto a 2014 Sierra SLT. I'm having issues with the low beams staying on when running. I used the 2018 body harness so I could directly plug the 2018 headlights in. Everything works EXCEPT for the low beams. When the truck is not running, I can turn on the low beams just fine. When I start the truck through, the low beams will come on and turn off after 1-2". I tried to wire in a resistor to each one and it made no difference. I'm wondering if I need an additional resistor, maybe 1 isn't enough? I also wired it to the yellow and ground wires respectively. I don't think it would matter if I wired it inline with the yellow wire only.

I bought the truck with 6,500 miles from CoPart. The engine harness was "fixed" in a couple of places and I chose to replace the entire harness. Prior to the replacement, I had the Fan 1 and Fan 2 lost comm codes which made sense since those wires were spliced. I also had the battery sensor code. After replacement for 2,500 miles I still had the Fan 1 comm code and battery sensor code. I assumed the battery sensor was bad since the original once was cracked and obviously bad. The Fan 1 I figured was a bad motor since Fan 2 began to work with the new harness. Then everything popped at once, the shutter, coolant pump, both fans, etc. So something common seems to have caused the issue. I left at 4 am and after stopping around 7 am I had a light and all associated codes listed above. It seems like it must be a relay or module for the body or chassis if I had to guess.

The main engine harness was just replaced with a brand new one from GM. After being replaced, I drove it 2,500 miles before the issues popped up. Current miles are about 12,500.

Neither of my cooling fans are working. A week ago only Fan 2 worked. However, after a quick stop last Tuesday, 200 miles into a 1,000 mile trip, I checked codes for an engine light and have the following: - U01B0: Lost Communication with Battery Sensor Module (had the previously and replaced the sensor, light came back anyway). - U0284: Lost Communication with Active Grille Air Shutter 1 Motor Module. - U0623: Lost Communication with Coolant Pump A - U0632: Lost Communication with Fan 1 (had previously, suspect maybe Fan 1 just failed but Fan 2 was working). - U0633: Lost Communication with Fan 2 - U1345: Engine Control Module LIN Bus 1 (had previously, not sure what this means exactly). I know these are all U codes so I'm guessing maybe a relay or module or something. This is a 2021 GMC Sierra Denali 1500. I also get a Service Suspension Message on the dash when I start the truck each time. I assume one of the shocks is likely bad, I believe it is the right rear. Any ideas where to start? I've checked wiring a few times already and don't see anything obvious.

I’ll check those and I have a couple of scan tools but nothing I need. I have a Zurich from HF but am trying to figure out which Tech 2 type scanner I should buy.

I should add, it will also pull the battery down slowly. I can watch it long enough to see it drop .1 volts every couple of minutes. So it seems like something is drawing constant power. While I expected some of the seats wouldn’t work, etc I’m surprised nothing has power, not even the headliner lights. I will start trying to make a list of every module I can find and try to isolate them one at a time. Hopefully someone can point towards one of them being the culprit.

Full disclosure, this is on a 2021 Yukon. I have a 2021 Sierra Denali sitting right next to it and have been swapping various things to troubleshoot with limited (really none) success so far. I bought this as a flood car and expected to have issues. It looks very clean on the inside but is essentially dead electronically everywhere. So far I replaced the junction box inside of the cab and have swapped the main fuse panel with the one off of the Sierra and have the same results. When I hook up the negative cable, I am getting some sounds out of the driver side firewall. I found a Spireon GPS tracker plugged into the OBD port and after removing it, I also get a couple of cycles of a small pump sound under the vehicle. I understand things like the seat, HVAC, radio, instrument cluster, etc may all be bad but I would be surprised if everything actually was. I have good power to everything from the battery to the power distribution on the positive side of the battery and have even replaced that with no change. I have good power to the main fuse box verified at the post. I've also swapped the entire fuse box with the Sierra along with the ECM and TCM (I know if won't actually work but worth a shot). The reason I think there is something obvious vs every single thing being bad, is that when I push the parking brake button, I can hear something under the vehicle (transmission area). I also have a small orange light in the button where you can turn the park assist on or off. I know the obvious answers are short to ground, bad ground, bad battery, etc. I've been chasing and searching for the culprit with no avail so far. Just curious what you think it might be and if anyone has experienced this. Again, this was a flooded vehicle and is a Yukon. I know how much more traffic this section gets and with my 2021 Sierra Denali sitting right next to it, I have verified they are essentially identical.