2019 GMC Acadia Engine Problems

Loss of steering assist code P0128 due to stuck thermostat.

Owner of a 2019 GMC Acadia Denali (VIN: [XXX] , current odometer ~87,164 miles), purchased new. In early February 2026, the check engine light came on. Independent diagnostic scan confirmed persistent P0420 code (“Catalyst System Efficiency Below Threshold Bank 1”) as current, history, pending, and permanent. Inspection showed the front exhaust pipe bracket had snapped, damaging the flex pipe and causing an exhaust leak. This failure mode directly triggers the P0420 code and requires replacement of the bracket and affected flex pipe section. This is a known recurring issue on GM’s Lambda-platform vehicles (shared platform, 3.6L V6 engine, and exhaust system design). GM Special Coverage Adjustment N212356540 covers 2019 Chevrolet Traverse and Buick Enclave models for front exhaust pipe bracket separation (resulting in pipe damage, holes, leaks, rattling, or efficiency codes), providing free repair for 15 years or 150,000 miles from in-service date, regardless of ownership. The failure on this Acadia is mechanically identical, yet the model is excluded from coverage despite matching part design and numerous similar owner complaints documented on NHTSA and BBB reports. Multiple attempts were made to resolve through the selling dealership (no response to service inquiries). Matter was escalated to GM Executive Resolution. GM required a redundant diagnostic scan at an authorized dealer (additional cost incurred), despite independent confirmation of the issue. GM ultimately denied any coverage, TSB applicability, goodwill assistance, or further support. The snapped bracket and resulting exhaust leak can produce noise, fumes in cabin, potential component damage, and emissions noncompliance via false/legitimate catalyst efficiency codes. Inconsistent application of coverage for the identical defect across equivalent Lambda-platform models raises concerns about a widespread safety/emissions issue affecting 2019 GMC Acadia. INFORMATION REDACTED PURSUANT TO THE FREEDOM OF INFORMATION ACT (FOIA), 5 U.S.C. 552(B)(6)

2019 GMC Acadia message: Action Required Steering Assist Is Reduced Drive with care Both power steering and AC affected, check engine light on, engine temperature gauge off

Check Engine Light came on with message of "Steering Assist is Reduced Drive with Care". This same thing happened to my sister-in-laws 2019 Acadia 2 months earlier. Took it in right away to the dealership to find out it's the thermostat, it's a common/known issue and it isn't covered under my extended warranty and will cost ~$1300. I'm seeing this is a common issue for Acadia's and has not had a recall, don't understand why. It's one thing if this is a easy part to replace, but GM has made it so that most people now need a dealership or auto repair shop to fix it.

Unknown My 2019 Acadia Denali has/is been repaired 6 times in 1 1/2 years for engine oil leaks , has had transmission repairs twice , radiator repair and replacement. Also suspension warranty work as well . I feel the oil leaks is and has been excessive and dangerous due to no warning lights never displayed to alert me to check oil

Shift to Park Vehicle does not recognize the gear shifter is in the park position. Vehicle does not fully shut down.

While driving the thermostat will go to zero and the steering assist is disabled. I have read a lot of forums that this is a known issue. The only way to reset it, so we have A/C is to disconnect the battery and it will last a day or two and do the same thing.

My 2019 GMC SLT seems to be having sensor problems. I keep getting a message that my driver assist is reduced and my A/C only blows out hot air. It’s done this to me randomly id say about 4-5 times. It will go off as I’m driving and than my temp gauge goes to zero. I did research and alot of other users have the same issue and say thermostat or thermostat sensor. If I disconnect my negative on battery and wait a few minutes it will work again.

My coolant thermostat went out, causing my AC and steering assist to stop working. I have spoke to multiple other people who have had this same issue with their GMC Acadia and I believe this should be a recall. It’s going to cost me $1200 to have this issue resolved.

Engine consuming extra oil due to failed piston ring. Also, may be the cause of a coolant failure code. Burning through excessive oil and check engine light came on. Also, prior to these warnings, I've felt hesitation when accelerating and when the car changes gears. I was told by a shop that I need a new engine. This causes a dangerous situation because the car could just stop working while driving or in traffic.

My 2019 GMC Acacia SLT would not go into park to allow me to shut off my vehicle. It ran for over an hour before it finally went into park. This was dangerous as I had a young child to pick up in a busy part of town and I had to leave it running to try to get help. With a 3 year old child this was crazy. This is not the first time it has done this. My car had a recall and whomever fixed it didn’t do it. I feel this needs to be fixed properly now.

The contact owns a 2019 GMC Acadia. The contact stated while driving at undisclosed speeds, the "Steering Assist - Reduce Speeds" message was displayed, and there was a loud roaring sound coming from the engine. The contact stated that the check engine warning light was illuminated. The failure recurred while the heat was turned off in the vehicle. The vehicle was taken to an independent mechanic where it was diagnosed with a faulty temperature gauge. The vehicle was not repaired. The manufacturer was not made aware of the failure. The failure mileage was approximately 80,000.

Thermostat gets stuck, generally in open state. AC also is disabled in this condition, soft code (no MIL) mentions "limited steering available" and temp gauge on dash goes to 0. Hard code eventually comes (solid Check Engine light on the dash). Code, MIL, and temp gauge can be cleared and fixed by disconnecting battery negative terminal and reconnecting it, and it will run consistently for a while, though it will return. Online there are no thermostat modules for this engine available (3.6L V6, also in Cadillac XT6, Buick Enclave, and Chevrolet Traverse) showing that thermostat replacement is widespread. Online forum searches also describe widespread thermostat issues for this engine, at roughly $1200 per fix. A reliable thermostat is critical for long-term engine functionality, and should not need replacement at half of design life (~75-80k miles). Though this one tends to fail open instead of close, overcooling of an engine still could have its own issues, and failing closed could lead to engine overheating.

I’ve had 2 issues with the thermostat going bad. The official part is called the water outlet assembly that includes the thermostat. I’ve also had several others tell me they have had this issue.

The contact owns a 2019 GMC Acadia. The contact stated that the check engine warning light was illuminated. The low oil message was displayed. The contact added oil to the engine. The vehicle was taken to the local dealer, where it was diagnosed that the oil gasket and plate needed to be replaced. The vehicle was not repaired. The manufacturer was not contacted. The failure mileage was approximately 72,885.

My vehicle would not turn completely off, I would have to start it about ten times before the warning sounds and the yellow light on the ignition switch would go off. I took my Acadia to Rod Hatfield Chevrolet Lexington, KY. I was told that there was an internal short with the transmission shifter, the shifter and harness would need to be replaced. considering that I purchased this vehicle new, and it has 39,793 miles I feel like it should have been covered under the warranty however, it cost me $903.3. I have an open case with GMC who suggested that I contact you to see if my Vehicle Identification Number could be added to for recall.

WHEN ACCELERATING THE VEHICLE EXHAUST OR ENGINE IS REALLY LOUD LIKE THERE'S AN EXHAUST LEAK.

DURING TAKEOFF WITH MODERATE ACCELERATION AROUND 1500-2000 RPM THERE IS AN EXHAUST DRONING/BUZZING SOUND FORWARD OF THE FRONT PASSENGER AREA. VEHICLE HAS BEEN TO DEALER TWICE, THE FIRST TIME THEY REPLACED THE ENTIRE EXHAUST SYSTEM, BUT AFTER 6 MONTHS THE SAME SOUND RETURNED. THE SECOND TIME THEY TRIED TO LOOSEN AND RETIGHTEN THE EXHAUST BUT THE SOUND RETURNED IMMEDIATELY AFTERWARD. THE DRONING SOUND IS MORE LIKELY TO OCCUR AFTER EXHAUST IS WARMED UP.

AT 500 MILES PAST THE WARRANTY EXPIRATION THE RADIATOR DEVELOPED A CRACK ALONG THE TOP SEAM. THE DEALERSHIP SAID IT WAS A MANUFACTURES DEFECT AS THE VEHICLE WAS LESS THAN TWO YEARS OLD. GMC DECLINED TO REPLACE THE PART BECAUSE IT WAS 500 MILES PAST THE WARRANTY ALTHOUGH THEY DID NOT DENY IT WAS A MANUFACTURING DEFECT. I HAVE A GMC CASE NUMBER DOCUMENTING MY DEALINGS WITH THEM. I HAVE PHOTOS FROM THE DEALERSHIP SHOWING THE CRACKED RADIATOR, AS WELL AS AN EMAIL STATING IT WAS A DEFECTIVE PART. I HAVE THE RECEIPT AND I HAVE THE ACTUAL RADIATOR ITSELF. I NOTICED THE RADIATOR LEAKING ON MY GARAGE FLOOR WHEN IT WAS STATIONARY. THIS WAS 10 DAYS AFTER IT WAS LAST SERVICED AND THERE WAS NO ISSUE AT THAT TIME. THE DEALERSHIP SAID IT WAS NOT CAUSED BY IMPACT, BUT JUST THAT THE SEAM SPLIT AT THE TOP OF THE RADIATOR.*DT*JB

I purchased my vehicle brand new off the truck in march of 2020 with zero miles on it. Feb 2021 in negative degree weather, the heat on my vehicle stopped working. I called my local GM dealership and they told me they needed to diagnosed the vehicle. They had kept my vehicle for two weeks trying to diagnose the vehicle. It turned out to be the thermostat. they replaced that part on my vehicle. Now Dec 2021 I again have experienced the same problem this time it was in a blizzard. Dealership told me it cant be the same problem, however it will be another two weeks before they can get my vehicle in for service and do not have a loaner vehicles for use. I purchased a new vehicle with warranty as I have two small children and am currently pregnant.