P0128 on a 2022 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 2022 Jeep Grand Cherokee

Ranked from 359 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Stuck-open thermostat
Medium severity
An owner's persistent cold-air-only heater complaint (tested repeatedly over weeks with no fix) is a classic symptom of a thermostat stuck open, keeping the engine below regulating temperature.
Reported in 1 NHTSA complaint
Parts:thermostat
2
Low coolant from a water pump leak
Medium severity
A water pump leaking antifreeze at 23,000 miles reduces system coolant volume, which can slow warm-up and skew temperature readings.
Reported in 1 NHTSA complaint
Parts:water pump
3
Low coolant from a recurring timing cover/head gasket leak
Medium severity
A documented casting defect caused a repeat timing-cover coolant leak even after dealer repair, another coolant-loss path that can trigger this code.
Reported in 1 NHTSA complaint
4
Failed or inaccurate ECT sensor
Medium severity
A drifting sensor can report a false low coolant temperature; verify against a second reading.
Common cause
Parts:engine coolant temperature sensor
5
Cooling fan stuck running continuously
Low severity
A fan that won't shut off over-cools the engine, preventing it from reaching regulating temperature.
Common cause
Parts:cooling fan relay
6
ECT sensor wiring/connector corrosion
Low severity
A high-resistance connection skews the coolant temperature signal low.
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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