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ENRUDEPLES

P0128 — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

minor

What it means

After a defined warm-up period and a calculated heat input, the coolant temperature remained below the value the thermostat should maintain. The ECM compares actual coolant temperature with a model based on run time, air flow, intake air temperature and vehicle speed, and sets this code when the engine warms up too slowly or cools down while driving. Practical effects are weak cabin heating, extended warm-up enrichment, higher fuel consumption and increased emissions.

Likely causes (most common first)

  1. Thermostat stuck open or with a wrong opening temperature
  2. Engine coolant temperature sensor reading low
  3. Coolant level low or system not properly bled
  4. Electric water pump or map-controlled thermostat heater circuit fault
  5. Cooling fan running permanently due to a stuck relay
  6. Wrong thermostat fitted during earlier repairs

Can you drive with it?

Driving is safe, but heating stays poor and consumption rises until the thermostat is replaced.

This is the generic meaning. What about your car?

The same code has different typical culprits on different engines. Tell the AI your make, model and year — it will rank the causes for your exact car and give you a check order.

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