P0128 on Kia: How to Diagnose & Fix
Coolant Thermostat Below Regulating Temperature
4 Kia models · 5 documented issues · 4 critical · $180-$800 repair
P0128 on Kia by model: symptoms, diagnosis and fix
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.
Common Symptoms
- Burning smell from engine bay
- Check engine light
- Oil pressure warning
- ISG system malfunction warning
- Engine stalling during restart after idle stop
How to Fix
Check VIN for Recall 23V-578. Dealer will inspect and replace the ISG oil pump assembly free of charge. Until the recall is completed, some owners disable ISG via the dashboard button to reduce risk.
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Owner tips & cautions
- TipPress the ISG off button each time you start the car to disable auto start-stop until the recall repair is completed
Sources for this issue (4)
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.
Common Symptoms
- ABS or other warning lamp may illuminate
- Burning odor, smoke or engine-compartment fire may occur
How to Fix
Check the VIN for campaigns 23V652000 and 22V051000. Until the applicable recall is completed, park outside and away from structures. A Kia dealer installs the campaign-specific HECU fuse protection free of charge. Do not diagnose this fire risk with P0217 or P0128 or buy wheel-speed sensors as a recall remedy.
Sources for this issue (2)
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.
Common Symptoms
- Smoke, melting odor or fire from the engine compartment may occur without advance warning
How to Fix
Check the VIN for recall 23V652000. Until repaired, owners are advised to park outside and away from structures. A Kia dealer replaces the HECU fuse free of charge. Do not diagnose this recall with P0217 or P0128, buy wheel-speed sensors, or promise HECU replacement from this page.
Sources for this issue (1)
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.
Common Symptoms
- Power-seat slide knob is stuck
- Burning odor, smoke or fire near a front power seat
How to Fix
Check the VIN for recall 26V430000/SC374 and park outside and away from structures until the repair is complete. A Kia dealer installs an electronic fuse assembly free of charge. Vehicles already repaired under 24V407/SC316 still need the new remedy. P0217 and P0128 are unrelated engine codes and are not part of this seat recall.
Sources for this issue (1)
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.
Common Symptoms
- Small coolant puddle under the front of the engine
- Low coolant warning with no visible large leak
- Sweet coolant smell after driving
- Crusty orange or green residue around the thermostat housing
- Drip from the water pump weep hole
- Temperature gauge creeping up in traffic
How to Fix
Pressure-test the cooling system cold to find the source before replacing parts - the housing leak often only shows under pressure. Replace the thermostat housing/water outlet as an assembly with a new gasket (the plastic flange cannot be trued); do not simply re-torque it, which cracks it further. A weeping water pump requires pump replacement with a new gasket; on the Nu engines the pump is belt-driven and accessible, keeping labor moderate. Refill with genuine Kia coolant and bleed properly. Not recall-covered.
Sources for this issue (4)
P0128 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0128 mean on a 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 it is documented across 4 models: Sportage, Sorento, Telluride, Forte.
What are the most common causes of P0128?
The most common causes of P0128 are: 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). The specific cause varies by vehicle.
How much does it cost to fix P0128 on a Kia?
Published repair estimates on Kia range from $180 to $800 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0128 on a different make?
View P0128 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.