DTC code
P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Technical guide
Symptoms
- Engine takes long to warm up
- heater not warm
- poor fuel economy
- temperature gauge stays low
- check engine light
Probable causes
- Thermostat stuck open (most common)
- faulty coolant temperature sensor
- faulty radiator fan running when not needed
Diagnostic steps
- Monitor temp during warmup.
- If plateau below 75°C after 15 min: thermostat stuck open - replace.
- Test ECT sensor readings.
- Check radiator fan operation.
Brand-specific notes (Ford)
Ford · Focus/Fiesta/Mondeo/Ka · all engines
Symptoms
- Engine takes long to warm up
- heater not very warm
- higher fuel consumption
- check engine light
- temperature gauge stays low
Probable causes
- Thermostat stuck open (most common)
- faulty coolant temperature sensor
- faulty ECT sensor giving false low reading
Diagnostic steps
- Monitor coolant temperature at operating temp - should reach 90-95°C.
- If temp never reaches 80°C after 10+ minutes, thermostat stuck open.
- Replace thermostat.
- If temp reads normally on dashboard but P0128 persists, replace ECT sensor.
Cross-checked technical synthesis (QA). It does not replace the manufacturer's procedure.
Models most reported with this code (NHTSA)
Count of distinct owner complaints mentioning the code. US data — same engines, same failures.
AI here is a librarian: this page shows only what mechanics confirmed, or says “unknown”. It never suggests a diagnosis — MECCANIC interprets the code, never your car.
0 confirmed cases on P0128
Nothing confirmed yet for this code. No guess is offered on purpose.
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