P0128 on a 2021 Kia Telluride

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 Kia Telluride

Ranked from 337 NHTSA owner complaints filed for this vehicle.

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1
Low coolant level from a system leak
Medium severity
Insufficient coolant volume slows warm-up and skews the sensor reading; this batch documents a Telluride with several simultaneous coolant hose and gasket leaks (heater core hoses, valve cover gaskets, upper/lower radiator hoses) reducing coolant in the system (odiNumber 11688419).
Reported in 1 NHTSA complaint
2
Cooling fan relay stuck on
Medium severity
A fan that runs when it shouldn't over-cools the engine and can prevent it from reaching thermostat-regulating temperature; this batch documents a Telluride with a dealer-diagnosed faulty cooling-fan relay causing the fans to run continuously (odiNumber 11720148).
Reported in 1 NHTSA complaint
Parts:cooling fan relay
3
Stuck-open thermostat
Medium severity
The single most common cause; coolant circulates through the radiator before the engine reaches operating temperature. Confirm with a scan tool warm-up graph that never reaches ~180-200F.
Common cause
Parts:thermostat
4
Failed or out-of-range coolant temperature (ECT) sensor
Low severity
Sends a falsely low reading to the PCM even though the engine is actually warm; check sensor resistance against a known-good curve.
Common cause
Parts:coolant temperature sensor
5
Trapped air pocket after a coolant-system service
Low severity
Air in the system prevents proper circulation and an accurate ECT reading until it's bled out.
Common cause
6
Corroded ECT sensor wiring or connector
Low severity
An intermittent open circuit at the connector reads as a continuously cold engine.
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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