P0128 on a 2021 GMC Sierra 1500

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 2021 GMC Sierra 1500

Ranked from 521 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Electronic coolant control valve (ECCV) failure
High severity
GM's electronically-controlled coolant valve used on the 5.3L/6.2L L87/3.0L Duramax engines sticks or fails, letting coolant flow erratically and keeping the engine cooler than the thermostat's regulating point; look for GM TSB 25NA-185, Special Coverage N252508341, and a P1098-family code alongside P0128.
Reported in 10 NHTSA complaints
Parts:coolant control valve
2
Cooling fan stuck running continuously
Medium severity
One owner reports the cooling fans continuing to run at high speed even with the truck shut off, which will keep coolant artificially cool.
Reported in 1 NHTSA complaint
Parts:cooling fan relay cooling fan
3
Stuck-open thermostat
Low severity
The wax-pellet or mechanical thermostat fails to close fully, letting coolant circulate before the engine reaches operating temperature.
Common cause
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Low severity
A drifting or shorted ECT sensor under-reports coolant temperature even though the engine is properly warmed up.
Common cause
Parts:engine coolant temperature sensor
5
Low coolant level from a leak
Medium severity
Insufficient coolant volume slows heat soak and can keep sensed temperature under the regulating threshold.
Common cause
6
Extended idling in cold weather
Low severity
Large-displacement V8 trucks that idle a lot in cold weather can struggle to generate enough heat to reach the thermostat's regulating point, especially with a marginal thermostat or ECCV.
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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