P0128 on a 2014 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 2014 Jeep Grand Cherokee

Ranked from 2,369 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Faulty Engine Coolant Thermostat
Low severity
The thermostat may be stuck open, preventing the engine from reaching its optimal operating temperature within the expected timeframe.
Common cause
2
Low Engine Coolant Level
Medium severity
Insufficient coolant can cause improper temperature readings or prevent the engine from warming up correctly.
Common cause
3
Defective Engine Coolant Temperature (ECT) Sensor
Low severity
A malfunctioning ECT sensor can send inaccurate temperature readings to the PCM, triggering the code.
Common cause
4
Cooling System Leaks
Medium severity
Leaks in the radiator, hoses, or water pump can lead to low coolant levels, affecting engine temperature regulation.
Common cause
5
Faulty Radiator Fan Relay
Low severity
If the cooling fan runs continuously due to a faulty relay, it can prevent the engine from reaching operating temperature.
Common cause
6
Intake Air Temperature (IAT) Sensor Malfunction
Low severity
An inaccurate IAT sensor can affect the PCM's calculation of the expected engine warm-up time.
Common cause

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.

Rather have a pro handle it? Find a vetted local mechanic.