P0128 on a 2016 GMC Yukon Denali Xl 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 2016 GMC Yukon Denali Xl 1500

Ranked from 497 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Stuck-open thermostat
Medium severity
A mechanic on this exact vehicle diagnosed the thermostat as stuck open, causing the engine to run cold/hesitate -- the single most direct match for P0128 in this batch.
Reported in 1 NHTSA complaint
Parts:thermostat
2
Failed or drifting ECT (coolant temperature) sensor
Low severity
One owner reported the "temperature sensor" going out (with the cooling fan running full speed) and needing repeated replacement -- a bad ECT/fan-control sensor can both under-report engine temp and trip P0128.
Reported in 1 NHTSA complaint
Parts:engine coolant temperature sensor
3
Low coolant level from an internal leak
Medium severity
This vehicle also has a documented pinhole coolant leak found under an injector after a tune-up; a system running low on coolant warms up more slowly and can set P0128.
Reported in 1 NHTSA complaint
4
Cooling fan stuck running
Low severity
A fan that won't cycle off keeps pulling heat out of the engine, preventing it from reaching its regulating temperature.
Common cause
5
Weak or failing radiator cap unable to hold system pressure
Low severity
A cap that can't hold pressure lets coolant boil off or vent early, lowering effective coolant volume and slowing warm-up.
Common cause
Parts:radiator cap
6
Trapped air pocket in the cooling system
Low severity
Air trapped near the ECT sensor after a coolant service or the pinhole-leak repair delays accurate temperature sensing and can trip a false low-temp 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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