P0128 on a 2017 GMC Acadia

P0128 means the engine warmed up slower than expected or stayed cooler than the computer expected.

Not your vehicle? See general P0128 info.

Most common causes on the 2017 GMC Acadia

Ranked from 701 NHTSA owner complaints filed for this vehicle.

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1
Stuck-open/failed thermostat housing assembly (3.6L LGX)
Medium severity
This is an extremely well documented defect on the 3.6L LGX in this complaint set - repeated reports of the temperature gauge stuck on cold, the radiator fan running continuously, A/C going warm, and a 'Steering Assist is Reduced' message caused by low coolant temperature; several owners had the thermostat replaced and one explicitly pulled code P0128.
Reported in 11 NHTSA complaints
Parts:thermostat thermostat housing
2
Failing/inaccurate coolant temperature sensor
Medium severity
Several of the same complaints describe the temperature sensor itself being replaced (sometimes repeatedly) alongside the thermostat, since the sensor is integrated into the thermostat housing on this engine and commonly fails or drifts together with it.
Reported in 3 NHTSA complaints
Parts:coolant temperature sensor
3
Cooling fan stuck running (fan control fault)
Low severity
Multiple owners report the radiator fan running at full speed continuously, which by itself can keep coolant below the thermostat's regulating temperature even with a good thermostat.
Reported in 1 NHTSA complaint
4
Low coolant level
Medium severity
Insufficient coolant volume prevents the engine from reaching and holding normal operating temperature; check for leaks and top off before condemning the thermostat.
Common cause
5
Thermostat housing gasket/seal leak
Low severity
A leaking housing gasket both loses coolant and can prevent the thermostat from seating and closing fully.
Common cause
Parts:thermostat housing gasket
6
Repeated short-trip/cold-weather driving that never lets the engine fully warm up
Low severity
Even on a healthy system, short trips in cold weather can keep coolant temperature below the monitor's threshold long enough to set the code; rule this out before replacing parts.
Common cause

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0128 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Weak cabin heat
Poor fuel economy
Temperature gauge low
Long warm-up time

What to check before replacing parts

Check coolant level when the engine is cold

Look for: Low coolant reduces the system's ability to reach and hold operating temperature — top off and recheck before assuming the thermostat or sensor is at fault.

Compare scan-tool coolant temperature to actual warm-up time

Look for: An engine that takes noticeably longer than normal to reach operating temperature (or never gets there) on the scan tool, with normal coolant level, points to the thermostat rather than the sensor.

Inspect thermostat operation

Look for: A thermostat stuck open (or opening at the wrong temperature) lets coolant circulate through the radiator too early, keeping the engine running cooler than it should — this is the most common cause of this code.

Check cooling fan command

Look for: A fan that runs constantly or turns on too early (commanded on when temperature is still low) keeps the engine from warming up normally, mimicking a stuck-open thermostat.

Inspect coolant temperature sensor data

Look for: A sensor reading that doesn't rise smoothly with actual engine warm-up, or that reads implausibly low compared to how the engine feels/behaves, points to a sensor or wiring fault rather than a true cooling problem.

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