P0128 on a 2018 Ford Explorer

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 2018 Ford Explorer

Ranked from 713 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Internal water pump failure / coolant loss (3.5L EcoBoost)
High severity
A large cluster of owners report the internal water pump failing and leaking, with coolant reservoirs running dry and needing repeated refilling — coolant loss and dilution keep the engine from reaching/holding thermostat-regulating temperature, setting P0128.
Reported in 8 NHTSA complaints
Parts:water pump water pump gasket/seal
2
Head gasket failure allowing coolant loss/intrusion
High severity
Owners report coolant disappearing into the cylinders (diagnosed as cylinder head/gasket failure), which drops coolant level and prevents the system from reaching operating temperature.
Reported in 2 NHTSA complaints
Parts:head gasket
3
Stuck-open thermostat
Medium severity
A thermostat that won't close lets coolant circulate through the radiator continuously, so the engine never reaches regulating temperature.
Common cause
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Medium severity
A drifting or failed ECT sensor reports a falsely low coolant temperature to the PCM even though the engine is actually up to temperature.
Common cause
Parts:engine coolant temperature sensor
5
Low coolant level from an undiagnosed leak (hose, radiator)
Medium severity
Insufficient coolant volume slows warm-up and can prevent the system from reaching thermostat-regulating temperature.
Common cause
Parts:radiator hose
6
Cooling fan stuck running continuously
Medium severity
A fan that won't shut off (relay or module fault) over-cools the engine bay and keeps coolant temperature below the thermostat setpoint.
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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