P0128 on a 2020 Jeep Wrangler

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 Wrangler

Ranked from 597 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant loss from water outlet/inlet housing bolt failure (documented TSB/recall on this platform)
High severity
A cluster of owners trace coolant leaks, an empty reservoir, or a leaking thermostat housing directly to water-inlet bolts backing out (TSB 07-001-22REVB / recall ZD8) -- a well-documented issue on this engine that starves the cooling system and can prevent normal warm-up.
Reported in 7 NHTSA complaints
Parts:water outlet housing coolant reservoir
2
Cooling fan stuck running continuously
Medium severity
One owner reported the engine fan staying on for years regardless of temperature, which would keep the engine from reaching or holding its normal operating temperature.
Reported in 1 NHTSA complaint
Parts:cooling fan cooling fan relay
3
Stuck-open thermostat
Medium severity
A thermostat that won't fully close lets coolant flow to the radiator before the engine is warm, keeping ECT below the P0128 threshold.
Common cause
Parts:thermostat
4
Failed or drifting engine coolant temperature (ECT) sensor
Medium severity
A sensor reading falsely low triggers the code even when the engine has actually reached temperature.
Common cause
Parts:engine coolant temperature sensor
5
Air pocket in the cooling system after a coolant top-off or repair
Low severity
Trapped air prevents proper coolant circulation past the ECT sensor and can delay the engine from showing it has reached temperature.
Common cause
6
ECT sensor wiring or connector corrosion
Low severity
A corroded connector or damaged wire can bias the temperature signal low, mimicking a true under-temperature fault.
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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