P0128 on a 2021 Ford Bronco Sport

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 2021 Ford Bronco Sport

Ranked from 665 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Recurring coolant loss from head gasket / cracked engine block
High severity
This 1.5L EcoBoost has a documented cracked-block/head-gasket defect causing repeated, unexplained coolant loss (air bubbles in the reservoir, coolant-in-oil at engine failure) — too little coolant mass delays reaching thermostat-regulating temperature.
Reported in 2 NHTSA complaints
2
Water pump failure leading to coolant loss
Medium severity
One owner's coolant reservoir ran dry after a water pump failure, directly reducing the coolant available to bring the engine up to temperature.
Reported in 1 NHTSA complaint
Parts:water pump
3
Stuck-open thermostat
Medium severity
A thermostat that won't close lets coolant circulate through the radiator continuously, keeping engine temperature below the regulating point.
Common cause
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Low severity
A drifted or open ECT sensor reports a falsely low reading to the PCM even though the engine is actually warm.
Common cause
Parts:engine coolant temperature sensor
5
Cooling fan stuck on
Low severity
A fan that won't disengage continues pulling heat out of the engine, delaying warm-up past the code's threshold.
Common cause
6
Low coolant from an external hose or radiator leak
Low severity
Any unaddressed coolant leak reduces the thermal mass needed to reach operating temperature promptly.
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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