P0128 on a 2016 GMC Yukon

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

Ranked from 265 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Stuck-open or slow-to-close thermostat
Medium severity
GM documented this exact symptom in a service bulletin; one owner's Yukon had a radiator leaking antifreeze from excessive thermostat cycling.
Reported in 1 NHTSA complaint
Parts:thermostat
2
Low coolant level from a leak
Medium severity
An owner's Yukon needed radiator replacement for a coolant leak; low coolant volume slows warm-up enough to trigger this code.
Reported in 1 NHTSA complaint
Parts:radiator
3
Failed engine coolant temperature (ECT) sensor
Medium severity
A drifting or failed ECT sensor reports a falsely low temperature even with a good thermostat; compare live data to actual radiator hose temperature.
Common cause
Parts:coolant temperature sensor
4
Cooling fan stuck running
Low severity
A fan that won't shut off (bad relay/module) over-cools the engine and keeps coolant below the thermostat's regulating temperature.
Common cause
5
Thermostat housing gasket leak
Low severity
A leaking housing gasket lets coolant escape and can introduce air into the system; look for coolant seepage at the housing.
Common cause
Parts:thermostat housing gasket
6
Air pocket in the cooling system after service
Low severity
Incomplete bleeding after a coolant flush or repair traps air that slows warm-up and can intermittently trigger this 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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