P0128 on a 2020 Chevrolet Blazer

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 2020 Chevrolet Blazer

Ranked from 211 NHTSA owner complaints filed for this vehicle.

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1
Stuck-open thermostat / faulty integrated thermostat-ECT housing (GM 3.6L V6 known defect)
Medium severity
An overwhelming number of owners report the same pattern on this engine: temp gauge pegged cold, AC disabled, radiator fans locked at full speed, and an explicit P0128 (or related P01F0/P0208) code traced to the thermostat, which GM dealers and TSBs acknowledge as a widespread 3.6L issue.
Reported in 23 NHTSA complaints
Parts:thermostat thermostat housing coolant temperature sensor
2
Radiator seam leak causing coolant loss
Medium severity
Two owners separately reported a leaking radiator seam/broken radiator alongside the cooling-system fault, which can starve the system of coolant and skew the temperature reading.
Reported in 2 NHTSA complaints
Parts:radiator
3
Failed/miscalibrated ECT sensor rather than the thermostat itself
Low severity
One detailed owner report noted the fault clears after disconnecting the battery and does not behave like a mechanically stuck thermostat, suggesting the PCM is misreading a bad coolant-temperature sensor even with a good thermostat.
Reported in 1 NHTSA complaint
Parts:coolant temperature sensor
4
Low coolant level
Low severity
Even a modest coolant deficit can keep the sensor reading below the thermostat's regulating threshold long enough to set the code.
Common cause
5
Cooling fan control fault (fan stuck running)
Low severity
A fan that won't cycle off continuously pulls heat out of the coolant, preventing the engine from reaching its normal operating temperature.
Common cause
Parts:cooling fan
6
Corroded ECT sensor wiring/connector
Low severity
A high-resistance or intermittent connection at the sensor connector produces an erratic low-temperature signal identical to a sensor failure.
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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