P0128 on a 2017 GMC Acadia Limited

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 Limited

Ranked from 639 NHTSA owner complaints filed for this vehicle.

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1
Stuck-open/failing thermostat (3.6L LGX — widely reported)
Medium severity
Multiple owners had the thermostat diagnosed and replaced after the engine ran cool, the radiator fan ran full-speed continuously, and dealers/forums described it as a very common failure on this 3.6L LGX engine.
Reported in 4 NHTSA complaints
Parts:thermostat
2
Failed/erratic coolant temperature sensor
Low severity
Several owners had the temperature sensor itself replaced (sometimes alongside the thermostat) after the temp gauge stopped reading correctly or the check-engine light traced back to a thermostat/sensor fault.
Reported in 3 NHTSA complaints
Parts:coolant temperature sensor
3
Grease/lubricant issue affecting thermostat operation (GM bulletin 17-NA-158)
Medium severity
Owners cited a known GM bulletin describing overly thick grease used in assembly that keeps the thermostat from operating properly, triggering a 'steering assist reduced' warning alongside the cooling fault.
Reported in 2 NHTSA complaints
Parts:thermostat
4
Cooling fan running continuously at full speed
Low severity
Several complaints described the radiator fan running abnormally loud/full-speed after the thermostat fault, which over-cools the engine and keeps it from reaching regulating temperature.
Reported in 2 NHTSA complaints
Parts:cooling fan
5
Low coolant level from an unrelated leak
Medium severity
Insufficient coolant volume can create an air pocket at the sensor and produce erratic low-temperature readings even with a good thermostat.
Common cause
Parts:coolant
6
Extended idling or short-trip driving in cold weather
Low severity
Little highway driving in cold conditions can prevent the engine from building enough heat load to satisfy the P0128 rationality check even on healthy components.
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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