P0128 on a 2015 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 2015 Ford Explorer

Ranked from 1,751 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Internal water pump failure on the 3.5L V6
High severity
The same chain-driven internal water pump defect seen across this model line: coolant leaks into the timing area, dropping coolant volume so the engine can't reach/hold operating temperature.
Reported in 8 NHTSA complaints
Parts:water pump
2
Low coolant level from a slow leak not fully bled after service
Low severity
Owner reported low coolant and an overheat-warning light with no visible leak, consistent with an under-filled or air-locked cooling system.
Reported in 1 NHTSA complaint
3
Stuck-open thermostat
Medium severity
The classic P0128 root cause: coolant bypasses the warm-up cycle and circulates through the radiator too early.
Common cause
4
Failed ECT sensor
Low severity
A sensor reading falsely low will trigger the code independent of actual coolant temperature.
Common cause
Parts:coolant temperature sensor
5
Cooling fan stuck on
Low severity
A fan that fails to cycle off keeps the radiator over-cooled and the engine below the P0128 threshold.
Common cause
Parts:cooling fan
6
Cracked thermostat housing
Medium severity
This plastic housing is prone to hairline cracks producing a slow coolant loss that mimics a water-pump leak.
Common cause
Parts:thermostat housing

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.