P0128 on a 2018 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 2018 GMC Acadia

Ranked from 389 NHTSA owner complaints filed for this vehicle.

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1
Stuck-open or failed thermostat
High severity
Extensively documented on this generation of Acadia: owners repeatedly cite the exact P0128 code, and dealers have replaced the thermostat/housing multiple times, with the failure also triggering a 'Steering Assist Reduced' warning.
Reported in 11 NHTSA complaints
Parts:thermostat thermostat housing
2
Faulty engine coolant temperature (ECT) sensor
High severity
Dealers on this vehicle replaced the coolant temp sensor together with the thermostat, and one owner's own OBD read pointed to 'a faulty Thermostat or Thermostat sensor.'
Reported in 2 NHTSA complaints
Parts:coolant temperature sensor
3
Cooling fan stuck running on high
Medium severity
One owner reported the radiator fan running on high constantly alongside the low coolant-temp/reduced steering-assist symptom, consistent with a fan that won't cycle off and over-cools the engine.
Reported in 1 NHTSA complaint
Parts:cooling fan relay
4
Low coolant level from a leak
Low severity
Insufficient coolant volume slows heat transfer to the sensor location, delaying the temperature reading.
Common cause
5
Trapped air pocket in the cooling system
Low severity
Air pockets after a coolant service can prevent proper circulation past the thermostat and sensor, delaying the reading.
Common cause
6
Extended idling or short-trip driving in cold weather
Low severity
Low engine load and cold ambient air alone can keep coolant below the regulating threshold long enough to set the code.
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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