What P0128 means
Termostato del refrigerante del motor (temperatura del refrigerante inferior a la temperatura de regulación del termostato)
How to diagnose
- Read freeze frame data. When P0128 set, your ECU snapshotted every sensor at that moment. This tells you what the car was doing when the fault triggered — RPM, load, coolant temp, throttle position, fuel trims. Almost always enough info to figure out the cause without further testing.
- Check for related codes. If you have P0128 plus other codes, fix the earliest one in the chain first — a single root cause often triggers multiple codes.
- Cheap fixes first. For most codes, the order is: inspect connectors + wiring (free), check for vacuum leaks (cheap), replace sensor (moderate), replace mechanical part (expensive). Follow that ladder before dropping money on parts.
Manufacturer-specific descriptions
Different manufacturers use P0128 for slightly different failure modes:
| AUDI | Defective Engine Thermostat, Defective Engine Coolant Temperature Sensor, Defective Intake Air Temperature Sensor, Defective Cooling System, Low Engine Coolant, Dirty Engine Coolant causing incorrect Coolant Temperature Sensor readings, Defective/always running Engine Cooling fan(s) |
| BMW | Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) |
| FORD | Coolant Thermostat Malfunction |
| HONDA | Cooling System Malfunction |
| MITSUBISHI | Coolant thermostat |
| VOLKSWAGEN | Coolant Thermostat |
Read P0128 on your own car
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