P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 28
- FaultThe specific fault in that area
What it means
P0128 means the engine coolant has not reached the temperature the thermostat should regulate to within the time the control unit expects. It compares the coolant temperature sensor reading against a modelled warm-up based on engine load, airflow, start temperature and time running. If the coolant warms too slowly or stays too cool, the code is set.
Symptoms
- Engine management warning light on
- Heater blows lukewarm or takes a long time to warm up
- Temperature gauge stays low or reads below normal when driving
- Fuel economy worse than usual
- Cooling fan running more than normal or constantly
- Engine seems slow to reach normal running behaviour on cold days
Possible causes
- Thermostat stuck open or opening too earlylikely
- Faulty or inaccurate coolant temperature sensor, giving a reading lower than actuallikely
- Wiring or connector fault on the coolant temperature sensor circuit, such as corrosion or high resistancepossible
- Low coolant level or air in the cooling system affecting sensor contact and heat transferpossible
- Cooling fan running continuously or when it should not, or a faulty fan controlpossible
- Incorrect or degraded coolant mixture, or wrong thermostat fitted previouslyrare
- Faulty intake air temperature sensor upsetting the warm-up model (depends on the model and engine)rare
How to diagnose
- Read stored and pending codes and note the freeze frame data, including coolant temperature, intake air temperature and engine load at the time the code set
- Check the coolant level and condition when the engine is cold, and look for leaks, contamination or signs of incorrect coolant
- Inspect the coolant temperature sensor connector and wiring for damage, corrosion or poor pin tension
- With a scan tool, watch live coolant temperature from a cold start and check it rises steadily to the normal operating temperature, typically around 90 degrees C, though this varies by model
- Compare the live coolant reading with an independent temperature measurement of the engine or thermostat housing to check sensor accuracy
- Check that the cooling fan is not running when the engine is cold and that its control operates correctly
- Feel the upper radiator hose during warm-up; if it heats up early while the engine is still cool, the thermostat is likely stuck open
- Check the intake air temperature sensor reading against ambient temperature on a cold engine, and check for any manufacturer service information relevant to the model and engine
How to fix
- Replace the thermostat (and housing or seals where required) with the correct specification for the engine
- Replace the coolant temperature sensor if its readings do not match actual temperature
- Repair damaged wiring or connectors in the sensor circuit
- Top up or replace the coolant with the correct type and mixture, and bleed air from the system following the manufacturer procedure
- Repair the fault in the cooling fan or its control circuit if it is running when it should not
- Replace the intake air temperature sensor if it is found to be reading incorrectly
Can I keep driving?
The car can normally be driven while you arrange a repair, as the engine is running cooler rather than overheating. Prolonged running too cool increases fuel use and engine wear, and a low coolant level should be corrected promptly.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.