P0128 on Kia
Coolant Thermostat Below Regulating Temperature
P0128 on Kia vehicles indicates coolant thermostat below regulating temperature. Au7o has documented this code across 4 Kia models — most commonly on Forte, Sorento, Sportage. This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions. Typical repair costs on Kia range from $180 to $800, depending on the specific model and root cause.
Common Causes of P0128
- •Thermostat stuck open or opening too early
- •Faulty engine coolant temperature sensor
- •Low coolant level
- •Cooling fan running constantly (overcooling)
- •Wrong-temperature-rating thermostat installed
- •Corroded or loose coolant temperature sensor connector
- •Air pockets in the cooling system
- •Faulty PCM/ECM (rare)
P0128 on Kia by Model
Kia Forte(1 issue)
- Plastic Thermostat Housing Cracking and Water Pump Seal Weep Causing Slow Coolant Loss2010-2025
Forte cooling systems across all three generations lose coolant from two recurring points. The plastic thermostat housing (water outlet) on the Nu engines warps or develops hairline cracks with thermal cycling, leaving a small puddle under the front of the engine that owners describe as appearing in cold weather and drying up in summer as the plastic expands and re-seals. Second, the water pump weep hole starts dripping as the shaft seal wears. Both are slow leaks, so the first symptom is usually a low-coolant light or a sweet smell rather than an overheat - but on a GDI Nu engine, running the coolant low is how a slow leak becomes a warped head. Forum threads on both forteforums.com and kia-forums.com track the housing leak specifically, with owners reporting warped housing flanges rather than a failed gasket.
Kia Sorento(1 issue)
- HECU Electrical Short and Engine Compartment Fire Risk2011-2014
NHTSA recall 23V652000 (Kia SC284) covers 2011-2014 Sorento vehicles. The Hydraulic Electronic Control Unit can experience an electrical short and cause an engine-compartment fire while parked or driving.
Kia Sportage(2 issues)
- Idle Stop & Go Oil Pump Overheating2023-2024
Certain 2023-2024 Sportage vehicles have an electronic controller in the Idle Stop & Go (ISG) oil pump assembly that may contain damaged electrical components. The damaged components can cause the oil pump to overheat, increasing the risk of a vehicle fire. NHTSA Recall 23V-578 covers affected vehicles. Kia dealers replace the ISG oil pump assembly.
- HECU Electrical Short / Fire Risk2012-2016
Two NHTSA recalls cover the frozen Sportage year set. Campaign 23V652000 (Kia SC284) covers 2010-2013 Sportage vehicles, including 2012-2013 here, and campaign 22V051000 (Kia SC227) covers 2014-2016. In both, an HECU electrical short can cause an engine-compartment fire while parked or driving.
Kia Telluride(1 issue)
- Front Power Seat Motor Overheating / Fire Risk2020-2024
NHTSA recall 26V430000 (Kia SC374) covers certain 2020-2024 Telluride vehicles. A front power-seat motor may overheat because of a stuck slide knob or an improper earlier 24V407 repair, causing a fire while parked or driving. This recall replaces 24V407.
Looking for P0128 on a different make?
View P0128 across all makes →Frequently Asked Questions
What does P0128 mean on Kia?▼
P0128 stands for "Coolant Thermostat Below Regulating Temperature." This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions. On Kia specifically, this code is documented across 4 models.
What causes P0128 on Kia vehicles?▼
Common causes on Kia: Thermostat stuck open or opening too early, Faulty engine coolant temperature sensor, Low coolant level, Cooling fan running constantly (overcooling), Wrong-temperature-rating thermostat installed. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0128 on a Kia?▼
Repair costs on Kia range from $180 to $800, depending on the specific model and root cause.
Which Kia models have P0128 documented?▼
Au7o has documented P0128 on 4 Kia models: Forte, Sorento, Sportage, Telluride.