Reference meaning
What does this code mean?
The warm-up performance did not meet the monitor's expectation. Check coolant level only when the engine is cool, compare temperature readings from a cold start, and assess the thermostat and cooling-fan operation. Climate, sensor accuracy and vehicle calibration matter. Never open a hot pressurised cooling system.
Sources and interpretation [1]
A code identifies a detected condition, not a part that must automatically be replaced.
Narrow by vehicle
A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Scan worksheet →Manufacturer wording
Source wording can vary by vehicle and suffix. It is shown as written, not as a universal diagnosis.
P0128Coolant Thermostat (Coolant Temp below Thermostat Regulating Temperature) logic revision
P0128Coolant Thermostat - Coolant Temp Below Thermostat Regulating Temperature
P0128Coolant Thermostat (Coolant Temp below Thermostat Regulating Temperature) logic improvement
Manufacturer communications
248Original wording in English
HYUNDAI
Certain Santa Fe Hybrid (TM HEV), Santa Fe Plug-in Hybrid (TM PHEV), Tucson Hybrid (NX4 HEV), andTucson Plug-in Hybrid (NX4 PHEV) vehicles may experience intake air line freezing and Exhaust GasRecirculation (EGR) valve corrosion due to excessive condensation accumulating within the EGR system.This condition can lead to drivabil…
HYUNDAI
Certain Santa Fe Hybrid (TM HEV), Santa Fe Plug-in Hybrid (TM PHEV), Tucson Hybrid (NX4 HEV), and Tucson Plug-in Hybrid (NX4 PHEV) vehicles may experience intake air line freezing and Exhaust Gas Recirculation (EGR) valve corrosion due to excessive condensation accumulating within the EGR system. This condition can lead to driva…
HYUNDAI
Certain Santa Fe Hybrid (TM HEV), Santa Fe Plug-in Hybrid (TM PHEV), Tucson Hybrid (NX4 HEV), and Tucson Plug-in Hybrid (NX4 PHEV) vehicles may experience intake air line freezing and Exhaust Gas Recirculation (EGR) valve corrosion due to excessive condensation accumulating within the EGR system. This condition can lead to driva…
SUBARU
This bulletin announces the availability of new reprograming files for the Engine Control Module (ECM) developed to remedy the following DTCs: P0016, P0017, P0018, P0019, P0014, P0024, P0128, P2109. If any of the DTCs listed above are found to be stored in the ECM, follow the procedures outlined in the bulletin. This is an revis…
Check vehicle applicability
839A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Sources and interpretation
These explanations are editorial summaries of the linked manufacturer material. PDF page numbers count from the first file page. Original publications are in English; translations are reference aids. A cited edition is not a claim that it is the manufacturer's latest instruction.
- 2024–2025 MY OBD System Operation Summary for Gasoline Engines ↗
Ford Motor Company · PDF page(s) 71 · 2024-04-16
Source check date · 2026-10-27
Can I keep driving?
A code alone cannot establish whether driving is safe. A flashing malfunction light, severe shaking, overheating, oil-pressure warning or loss of braking or steering needs prompt attention. Stop in a safe place and seek assistance when control or engine safety is affected. Follow the vehicle handbook.