P0128 on a 2016 GMC Yukon 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 Xl 1500

Ranked from 450 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Stuck-open thermostat
High severity
A dealer directly diagnosed a thermostat stuck open on this exact vehicle after it hesitated and ran poorly.
Reported in 1 NHTSA complaint
Parts:thermostat
2
Failed or drifting engine coolant temperature (ECT) sensor
Medium severity
One owner had the temperature sensor fail outright, causing the cooling fan to run at full speed continuously.
Reported in 1 NHTSA complaint
Parts:coolant temperature sensor
3
Cooling fan stuck running
Medium severity
The same failed-temperature-sensor case kept the cooling fan running on full speed, which prevents the engine from reaching or holding operating temperature.
Reported in 1 NHTSA complaint
4
Low coolant level
Medium severity
Insufficient coolant volume slows warm-up and can cause erratic ECT readings.
Common cause
5
Corroded ECT sensor wiring/connector
Low severity
Poor contact at the sensor connector can produce an erratic or falsely-low temperature signal.
Common cause
6
Air pocket in cooling system after service
Low severity
Trapped air prevents proper coolant circulation past the ECT sensor and thermostat, giving a false low-temp reading.
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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