P0128 on a 2019 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 2019 GMC Acadia

Ranked from 374 NHTSA owner complaints filed for this vehicle.

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1
Stuck-open thermostat
Medium severity
Owners overwhelmingly describe a thermostat that won't close, dropping the temperature gauge to zero and triggering the 'Steering Assist Is Reduced' warning because the power-steering module shares logic with the cooling system on this platform; widely reported as a known recurring issue on the 2019 Acadia.
Reported in 13 NHTSA complaints
Parts:thermostat
2
Failed engine coolant temperature (ECT) sensor
Medium severity
A drifting or dead ECT sensor reports a temperature that never reaches the thermostat's regulating point even when the thermostat itself is fine; one owner explicitly ties DTC P0128 to a coolant temperature sensor failure.
Reported in 2 NHTSA complaints
Parts:coolant temperature sensor
3
Cooling fan stuck running continuously
Low severity
A fan commanded on at high speed all the time (relay or PCM fault) over-cools the engine below the thermostat's regulating threshold; several owners describe the fan running loud and constant during the same fault event.
Reported in 2 NHTSA complaints
Parts:cooling fan relay
4
Low coolant level from a leak
Medium severity
Insufficient coolant volume slows warm-up and can prevent the engine from reaching the thermostat's regulating temperature; check the reservoir and pressure-test the system.
Common cause
5
Thermostat housing crack or seal leak
Medium severity
A leaking housing lets coolant escape and air enter the system, both of which delay reaching normal operating temperature.
Common cause
6
Corroded or damaged ECT sensor wiring/connector
Low severity
A poor connection at the sensor produces an erratic low-temperature signal that reads as an under-temperature fault even with a healthy thermostat.
Common cause
Parts:coolant temperature sensor

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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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