P0128 on a 2013 Ford Fusion

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 2013 Ford Fusion

Ranked from 1,948 NHTSA owner complaints filed for this vehicle.

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1
Low coolant level from chronic coolant loss (widespread head-gasket / block coolant-intrusion issue on this 1.6L EcoBoost)
High severity
Numerous owners report the coolant reservoir running low or empty, warning lights for low coolant, and repeated top-offs; insufficient coolant volume prevents the engine from holding a stable regulating temperature and can cause erratic ECT readings.
Reported in 13 NHTSA complaints
2
Erratic/failed engine coolant temperature (ECT) sensor
Medium severity
One owner had the coolant temperature sensor replaced after conflicting over-temp warnings with no fault found on the OBD system, consistent with a sensor giving implausible readings.
Reported in 1 NHTSA complaint
Parts:engine coolant temperature sensor
3
Stuck-open thermostat
Medium severity
A thermostat that fails to close lets coolant circulate continuously through the radiator, keeping engine temperature below the regulating point; confirm with scan-tool warm-up time versus a known-good curve.
Common cause
Parts:thermostat
4
Cooling fan stuck running continuously
Low severity
A fan relay or fan control module fault that keeps the electric fan on over-cools the engine even at operating temperature.
Common cause
Parts:cooling fan assembly fan control module
5
Air pocket in the cooling system after a top-off or repair
Low severity
Trapped air prevents the ECT sensor from reading true coolant temperature until the system is properly bled, common after the frequent coolant refills reported on this vehicle.
Common cause
6
Corroded or damaged ECT sensor wiring/connector
Low severity
Poor connector contact biases the sensor signal toward a low-temperature reading; inspect and clean the connector before replacing the sensor.
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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