P0128 on a 2020 Jeep Gladiator

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 2020 Jeep Gladiator

Ranked from 478 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Low coolant from head gasket/cylinder head coolant loss
High severity
Reported head gasket failure and a cracked cylinder head caused ongoing coolant loss on this platform; low system coolant is a direct, well-known cause of P0128.
Reported in 2 NHTSA complaints
Parts:head gasket cylinder head
2
Thermostat failure/erratic operation
Medium severity
A dealer replaced the thermostat after the temperature gauge fluctuated wildly (normal to extremely high) and the engine idled roughly; a thermostat that isn't seating or opening/closing correctly can also let coolant temp read below the PCM's expected regulating point.
Reported in 1 NHTSA complaint
Parts:thermostat
3
Failed engine coolant temperature (ECT) sensor
Low severity
A drifting or failed ECT sensor reports an inaccurately low coolant temperature to the PCM even though the engine is actually warm.
Common cause
Parts:coolant temperature sensor
4
Cooling fan stuck on
Low severity
A fan that won't shut off over-cools the engine, keeping coolant temperature below the regulating threshold.
Common cause
5
Air pocket in the cooling system after repeated coolant top-offs/repairs
Low severity
Trapped air from the multiple cooling-system repairs reported on this platform can cause erratic temperature readings until properly bled.
Common cause
6
Extended idling or short-trip driving in cold weather
Low severity
Short trips or long idling in cold weather may not generate enough heat for the engine to reach its regulating temperature in time.
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.

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