P0128 on a 2013 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 2013 Jeep Grand Cherokee

Ranked from 799 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Stuck-open or failed thermostat
Medium severity
Directly reported on this vehicle/year: one owner's scan explicitly listed a 'thermostat rationality' fault, and others had the thermostat replaced for overheating/temperature complaints.
Reported in 3 NHTSA complaints
Parts:thermostat
2
Low coolant / coolant leak (radiator, hoses)
Medium severity
Reported radiator, hose, and general coolant-system issues on this vehicle reduce coolant volume/flow, slowing warm-up.
Reported in 1 NHTSA complaint
Parts:radiator coolant hose
3
Cracked oil filter/oil cooler housing (known 3.6L Pentastar issue)
Medium severity
One owner had the plastic oil filter housing crack and leak during thermostat work, a documented weak point on this V6 that can also let coolant escape.
Reported in 1 NHTSA complaint
Parts:oil filter housing oil cooler
4
Failed engine coolant temperature (ECT) sensor
Low severity
A drifted or failed ECT sensor can report 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 a coolant top-off or repair leaves air in the system, producing erratic warm-up behavior.
Common cause
6
Cooling fan stuck running
Low severity
A fan that runs continuously 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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