The fault code P0128 indicates a problem with engine temperature regulation, often related to a thermostat stuck in the open position. This malfunction prevents the engine from reaching its optimal temperature, impacting performance, fuel consumption, and longevity. Discover how to identify, repair, and avoid risks at a lower cost.
Symptoms of a Stuck Open Thermostat (P0128)
- 🚨 Engine temperature too low (needle stays below 90°C).
- 🚨 Weak cabin heating (air not very hot).
- 🚨 Increased fuel consumption (+10 to 20% in city driving).
- 🚨 Check engine light on (P0128 code stored).
- 🚨 Increased pollutant emissions (catalytic converter less efficient).
Risks of Ignoring a P0128 Code
- 🔥 Premature engine wear: Oil that is too cold does not lubricate parts properly.
- 🔥 Catalytic converter damage: The air-fuel mixture remains rich, overloading the catalytic converter.
- 🔥 Battery strain (hybrid vehicles): The engine runs longer to compensate for the increased consumption.
Causes of the P0128 Code
- Faulty thermostat (broken spring or mechanical wear).
- Damaged thermostat gasket (coolant leak).
- Faulty temperature sensor (sends incorrect data to the PCM).
Cost-Effective Repair in 4 Steps
1. Confirm the Diagnosis
- Use an OBD2 scanner to verify the P0128 code.
- Check the actual engine temperature via the diagnostic tool (compare with PCM data).
2. Test the Thermostat
- Start the cold engine.
- Touch the top of the radiator and the thermostat outlet hose: if these parts heat up immediately, the thermostat is stuck open.
3. Replace the Thermostat (Cost: €50 to €150)
- DIY:
- Partially drain the coolant.
- Remove the thermostat housing (depending on the model).
- Install a new thermostat (check the opening temperature, e.g., 87°C).
- Refill and bleed the cooling system.
- Garage: Allow 1 to 2 hours of labor (average cost: €80 to €150).
4. Check the Temperature Sensor
- Measure its resistance with a multimeter (expected values: ~2000Ω when cold, ~300Ω when hot).
FAQ: Saving on P0128 Repair
Q: Can you drive with a stuck open thermostat?
Yes, but avoid long trips: increased consumption and accelerated wear are to be expected.
Q: OEM or aftermarket thermostat?
Prefer brands like Gates, Wahler, or Febi (€15 to €40), as reliable as manufacturer parts.
Q: P0128 code recurring after replacement?
Check the coolant (level, quality) and the ECT sensor.
Conclusion
A P0128 code should not be ignored, but its repair remains affordable. By replacing the thermostat yourself or opting for an independent garage, you can limit costs to less than €150 in most cases. Act quickly to preserve your engine and avoid more serious breakdowns!