P0128 on a 2018 Jeep Grand Cherokee Srt

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 2018 Jeep Grand Cherokee Srt

Ranked from 685 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Head gasket failure causing coolant loss
High severity
Several owners had head gaskets blow with combustion gases entering the cooling system, which both loses coolant and can confuse the ECT reading.
Reported in 3 NHTSA complaints
Parts:head gasket
2
Water pump design flaw / leak causing low coolant
High severity
One complaint specifically flags a water pump design flaw producing persistent coolant leaks, and repeated coolant top-offs on another vehicle point to the same slow-leak pattern that keeps ECT below the thermostat threshold.
Reported in 2 NHTSA complaints
Parts:water pump
3
Stuck-open thermostat
Medium severity
A thermostat stuck open circulates coolant through the radiator continuously, so the engine never warms up within the expected time.
Common cause
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Medium severity
A drifting or biased sensor can report a temperature below the regulating threshold even when the engine is actually warm.
Common cause
Parts:engine coolant temperature sensor
5
Cooling fan stuck running
Low severity
A fan that won't shut off continues pulling heat from the coolant, delaying or preventing warm-up.
Common cause
Parts:cooling fan
6
Low coolant from a minor hose/radiator leak
Medium severity
Any slow external coolant leak reduces system volume enough to slow warm-up past the code's time threshold.
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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