2018 GMC Sierra 1500

Trailer brake modulator on 2018 GMC Sierra does not consistently signal trail brakes to work when needed on our 7,500 pound RV trailer. GMC dealer eventually found that the battery cables were undersized as installed with the factory towing package on the 2018 GMC Sierra. The undersized battery cables do not consistently deliver adequate voltage to the trailer brake modulator on the GMC Sierra, so inconsistent electric brake function for our 7,500 pound RV trailer. Dealer had difficulty diagnosing the problem as being the wrong factory-installed battery cables to go with the factory-installed trailer towing package. So it cost us $1,600 to have the GMC dealer correct a problem caused by the GMC factory installing undersized battery cables. Fortunately, this did not result in a collision for us, but it might for someone else. Our 2018 GMC Sierra kept sending vague, randomly timed warnings to the dashboard to "Check Trailer Wiring." Very frightening, dangerous and costly (for us) mistake by GMC. Will the brakes work this time, or fail on the way home? Problem started in 2021 RV season, but took awhile for us to recognize the need to take to the GMC dealer for them to diagnose base cause. Took time for us to experiment changing trailer wiring connector on bumper and trailer brake control on the dash. Work for awhile, then fail. Work for awhile, then fail. Arrrrgh! Ask GMC to put more adequate battery cables on ALL their vehicles, please.

I have a 2018 GMC Sierra Denali. I started having problems with the 8-speed transmission at 18,000 miles. The vehicle has been to the dealership for a transmission fluid flush, a transmission relearn, a 5-week valve body replacement, two more trips to the dealership where the truck did not malfunction for them. I have had issues with an air leak on the driver side front door as well. That issue has not been resolved as well. I spent over $60,000 for the truck and have had nothing but problems. I have brought it to General Motors attention and all I would do is get a run around and no results. I was told that there is nothing more that they can do. I am beside myself and just sick as I do not know what to do. I bought a brand new vehicle in order to not have problems compared to a used one.

I was driving down the highway when my vehicle loss control of operation and check engine light came on. After taking it to the GMC dealership it was determined to be a bad lifter in the motor. The garage had to take the motor completely apart to repair a bent rod and cylinder in the motor. There are numerous reports and a pending class action lawsuit concerning this device in the GMC Sierra trucks and other GMC vehicles. The lifters in the cylinder of this motor are designed to switch the engine from V4 to V6 to save on gasoline. My vehicle has been repaired by the GMC garage but I've seen reports of this problem repeatedly happening in these vehicles and it causes me great concern because if there is a lot of traffic in the area I could get hit or cause an accident by not being able to operate the truck. My vehicle is available for inspection upon request. The safety of myself and others is put at risk because the sudden equipment breakdown could cause an accident. The problem was confirmed by the dealer. No other parties have inspected the vehicle. No warning lamps went off prior to this happening and the vehicle is only a 2018 and only has 36,000 miles on it.

The left rear tail light assembly is subject to direct wheel spray resulting in premature corrosion and failure of the lightbulb and wiring harness. Failure first occurred Within 18 months of ownership. I cleaned the contacts and packed the connection with dielectric grease. However after three years the bulb connection is now totally corroded and the circuit has failed requiring harness replacement. The problem is so widespread, replacement wiring harnesses for the left rear tail light assembly are out of stock and unavailable from GM parts. I am now using the vehicle without operating right rear turn signals. GM-trucks.com is an owners chat room with many entries about this problem. [XXX], BSME, P.E. SAE INFORMATION Redacted PURSUANT TO THE FREEDOM OF INFORMATION ACT (FOIA), 5 U.S.C. 552(B)(6).

On 2/26/22, approximately 24,000 miles on the vehicle, while traveling on the Interstate a warning popped up in the display that the power steering had failed and to operate the vehicle with caution. Immediately I noticed I had no power steering, and the vehicle was extremely hard to steer. I exited the interstate and turned the vehicle off. I conducted a visual inspection; no issues were immediately apparent. After several minutes I started the vehicle, and the power steering began functioning. I conducted an Internet search and learned this is a common issue for GMC trucks, previously resulting in numerous crashes and over one million vehicles being recalled. NHTSA campaign number: 18V586, GM’s number for the recall 18289. 3/1/22, I contacted GM customer care and was advised my vehicle was not part of the recall. I was advised to take the vehicle to a GM service center, at my cost. 3/1/22, I contacted Mossy GMC of Picayune, MS, and spoke with a technician, who advised the issue is usually due to a drop in voltage and advised me to replace the battery. 3/19/22, replaced the battery. 3/30/22, upon startup the power steering again failed. I turned the vehicle on and off several times, and the power steering began functioning. 3/30/22, went to Mossy of Picayune, spoke with a technician, who advised he was quite familiar with the issue, stating this is a common problem with GMC trucks, he advised to replace the battery. Told him I replaced the battery on 3/19/22, he stated I would likely need to have the steering module reprogrammed, at my expense $130.00, or replace the steering assembly, again at my expense $1,500.00. 4/3/22, while turning the power steering failed, and the vehicle was extremely difficult to steer. I turned the vehicle off and waited several minutes. Upon startup the power steering began functioning. The power steering failure is a MAJOR safety hazard and a recall needs to be issued before someone loses their life.