P0128 — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Category: Powertrain Severity: Medium
Updated:
DTC code P0128 means: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). It falls under the "Powertrain" category, with an estimated severity of Medium. Common causes include thermostat stuck open, low coolant level and faulty ECT sensor; it's worth confirming the diagnosis before replacing parts.
Possible causes
- Thermostat stuck open
- Low coolant level
- Faulty ECT sensor
- Faulty cooling fan
Symptoms
- Loss of power when accelerating
- Unstable idle
- Higher fuel consumption
- Jerking while driving
How to diagnose it step by step
- Check the coolant level.
- Log temperature on the scan tool: it should reach 185–205 °F in 10–15 minutes.
- If it takes much longer, the thermostat is stuck open.
- Compare the sensor reading with a thermometer.
- Replace the thermostat (and the sensor if needed).
How to fix it
Clean or replace the MAF/MAP sensor and check for intake leaks.
Estimated repair cost
Thermostat installed: $150–400.
Frequently asked questions
What does DTC code P0128 mean?
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).
Is code P0128 serious?
Severity Medium. Not urgent, but worth diagnosing to avoid bigger damage.
Can I keep driving with code P0128?
Yes, you can drive; fuel use will be higher and cabin heat weak.
Related codes
P0125 P0123 P0122 P0121 P0120 P0130 P0131 P0132 P0133 P0134
Refine by car make
- Audi P0128: Thermostat or temperature sensor.
- BMW P0128: Thermostat (typical failure).
- Citroën P0128: Thermostat or temperature sensor.
- Dacia P0128: Thermostat or temperature sensor.
- Fiat P0128: Thermostat.
- Ford P0128: Thermostat stuck open; on some models also the ECT sensor.
- Honda P0128: Thermostat or ECT sensor.
- Hyundai P0128: Thermostat.
- Kia P0128: Thermostat.
- Mazda P0128: Thermostat.
- Mercedes-Benz P0128: Thermostat.
- Nissan P0128: Thermostat.
- Opel P0128: Thermostat.
- Peugeot P0128: Thermostat or temperature sensor.
- Renault P0128: Thermostat or temperature sensor.
- Seat P0128: Same as VW: thermostat or ECT sensor.
- Škoda P0128: Same as VW: thermostat or ECT sensor.
- Toyota P0128: Thermostat stuck open.
- Volkswagen P0128: Thermostat or coolant temperature sensor.
- Volvo P0128: Thermostat.
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