P0128 on GMC ACADIA — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Q: "What does P0128 mean on a GMC ACADIA?"

"P0128 indicates: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). This is a fuel and air metering code. 10 GMC ACADIA owners have reported this to NHTSA."

Q: "What are the most common causes of P0128 on GMC ACADIA?"

"Based on 10 NHTSA reports, the most commonly cited components are: Steering,Power Train,Electrical System, Steering,Electrical System,Engine, Steering,Forward Collision Avoidance. These percentages are based on the component field in owner complaints."

Q: "Is P0128 serious on my GMC ACADIA?"

"Of 10 NHTSA reports for this code on GMC ACADIA, none involved a crash or fire. However, any diagnostic code should be evaluated by a qualified mechanic."

Q: "What should I do if my GMC ACADIA shows P0128?"

"1) Check for recalls on your VIN at nhtsa.gov/recalls. 2) Get a professional diagnosis — a code alone doesn't identify the exact failed part. 3) Get 2-3 repair estimates to compare."

What P0128 Means on Your GMC ACADIA

P0128 is a diagnostic trouble code indicating: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). This code relates to the fuel and air metering system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.

10 GMC ACADIA owners have reported this issue to NHTSA across model years 2017-2021. Most reports come from the 2017, 2020 model years.

Real GMC ACADIA Owner Reports

Source: NHTSA Complaints Database

"On [XXX] my 2017 Acadia Denali started driving poorly and a steering assist reduced message appeared. I had O'Reilly scan for error messages and they were: P0128 and P0826-00. The dealership confirmed that they would need to replace the thermostat."

— 2017 GMC ACADIA owner · STEERING,FORWARD COLLISION AVOIDANCE · Filed 01/31/2026

"Within a day of taking possession of the car, September 20, 2024, with odometer reading 76,352 , warning message in instrument cluster appears stating action required “steering assist is reduced, drive with care”. Following day, A/C stop working while driving."

— 2017 GMC ACADIA owner · STEERING,UNKNOWN OR OTHER,ENGINE · Filed 09/20/2024

"P0128 Apparently what happens when the temp sensor on the T-Stat housing goes bad it sends a signal to the ECM that the engine is overheating, causes the temp gauge to peg to "C", shuts down A/C Compressor Clutch engagement to not exacerbate the Overheat condition and super speeds the cooling fans."

— 2017 GMC ACADIA owner · STEERING,POWER TRAIN,ELECTRICAL SYSTEM · Filed 12/10/2022

"Seemingly at random a warning will pop up on the driver dashboard “Steering assist is reduced at this time”. This will lead to a engine coolant temp reading of 0 degrees, no a/c and reduced steering functionality."

— 2018 GMC ACADIA owner · STEERING,ELECTRICAL SYSTEM,ENGINE · Filed 07/07/2022

"The contact owns a 2019 GMC Acadia. The contact stated that while driving at an undisclosed speed, the message "Steering Assist Fault - Drive with Caution" was displayed."

— 2019 GMC ACADIA owner · STEERING,ENGINE AND ENGINE COOLING · Filed 10/25/2025

All reports filed by vehicle owners directly with the U.S. Department of Transportation.

📊 Complaint Trend by Model Year

3

'17

1

'18

2

'19

3

'20

1

'21

Common Causes on GMC ACADIA

Based on NHTSA complaint component analysis for this vehicle.

STEERING,POWER TRAIN,ELECTRICAL SYSTEM 20%

STEERING,ELECTRICAL SYSTEM,ENGINE 20%

STEERING,FORWARD COLLISION AVOIDANCE 10%

STEERING,UNKNOWN or OTHER,ENGINE 10%

STEERING,ENGINE and ENGINE COOLING 10%

What To Do Next

1
Check for recalls on YOUR VIN
Go to nhtsa.gov/recalls and enter your VIN. If your vehicle is covered, repairs are free.
2
Get a proper diagnosis
A code alone doesn't tell you the exact failed part. A diagnostic at a shop ($50–$150) pinpoints the root cause before you spend money on parts.
3
Compare repair quotes
Get 2–3 quotes. Dealer vs. independent shop prices often differ 30–50% for the same repair.

Frequently Asked Questions

What does P0128 mean on a GMC ACADIA? ▾

P0128 indicates: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). This is a fuel and air metering code. 10 GMC ACADIA owners have reported this to NHTSA.

What are the most common causes of P0128 on GMC ACADIA? ▾

Based on 10 NHTSA reports, the most commonly cited components are: Steering,Power Train,Electrical System, Steering,Electrical System,Engine, Steering,Forward Collision Avoidance. These percentages are based on the component field in owner complaints.

Is P0128 serious on my GMC ACADIA? ▾

Of 10 NHTSA reports for this code on GMC ACADIA, none involved a crash or fire. However, any diagnostic code should be evaluated by a qualified mechanic.

What should I do if my GMC ACADIA shows P0128? ▾

1) Check for recalls on your VIN at nhtsa.gov/recalls. 2) Get a professional diagnosis — a code alone doesn't identify the exact failed part. 3) Get 2-3 repair estimates to compare.