P0128 on a 2016 Hyundai Tucson Fuel Cell

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 2016 Hyundai Tucson Fuel Cell

Ranked from 1,536 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Water pump failure disrupting normal coolant circulation
Medium severity
A dealer diagnosis on this vehicle found the water pump faulty (along with other components) after a loss-of-power/vibration event, and another owner reported recurring water pump/hose failures.
Reported in 2 NHTSA complaints
Parts:water pump
2
Coolant hose failure
Low severity
Owner reported recurring blown coolant hoses on this vehicle, a source of both coolant loss and disrupted circulation.
Reported in 1 NHTSA complaint
Parts:coolant hose
3
Stuck-open thermostat
Medium severity
The thermostat fails to close and the engine cannot reach or hold its regulating temperature, especially noticeable on cold starts and highway cruise.
Common cause
Parts:thermostat
4
Failed or inaccurate engine coolant temperature (ECT) sensor
Medium severity
A drifting or failed ECT sensor reports a temperature that never reaches the commanded threshold even though the engine is actually warm.
Common cause
Parts:coolant temperature sensor
5
Low coolant level from a leak
Medium severity
Insufficient coolant volume slows warm-up and can prevent the sensor from ever reading full operating temperature.
Common cause
6
Cooling fan stuck running / stuck-on relay
Low severity
A fan that runs continuously (even key-off) over-cools the engine and can prevent it from reaching normal operating temperature.
Common cause
Parts:cooling fan

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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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