P0128 on a 2017 Jeep Grand Cherokee

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 2017 Jeep Grand Cherokee

Ranked from 361 NHTSA owner complaints filed for this vehicle.

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1
Low coolant from cooling-system leaks (radiator, EGR cooler)
Medium severity
Owners report the radiator not functioning properly and a cracked EGR coolant compartment leaking coolant on this vehicle, both of which reduce coolant volume and slow warm-up.
Reported in 2 NHTSA complaints
Parts:radiator EGR cooler
2
Chronic cooling-system issues on the 3.6L engine
Medium severity
A mechanic told one owner the 3.6L engine has repeat overheating issues; unstable cooling-system operation is consistent with intermittent below-threshold readings.
Reported in 1 NHTSA complaint
3
Stuck-open or failed thermostat
Medium severity
One owner's list of cold-weather symptoms specifically calls out the thermostat; a thermostat stuck open is the classic direct cause of this code, especially noticeable below freezing.
Reported in 1 NHTSA complaint
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Low severity
A drifted sensor reports a false low temperature independent of actual thermostat operation.
Common cause
Parts:coolant temperature sensor
5
Air pocket trapped in the cooling system
Low severity
Incomplete bleeding after any coolant leak repair leaves trapped air, causing erratic warm-up readings.
Common cause
6
Cooling fan stuck running
Low severity
A fan that won't shut off prevents the engine from reaching thermostat regulating temperature.
Common cause
Parts:cooling fan relay

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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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