P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
- Severity
- Low
- Driving
- No driving restriction
The engine coolant is not reaching the temperature the module expects within the expected time. It is not overheating - the opposite. The engine is running cooler than designed, which usually means the thermostat is stuck open and letting coolant circulate through the radiator before the engine has warmed up.
Possible causes
- CommonThermostat stuck open
- PossibleFaulty coolant temperature sensor reading low
- PossibleLow coolant level
- RareCooling fan running constantly when it should not
What you might notice
- Heater blows cool or takes a long time to warm up
- Temperature gauge reads lower than normal or takes a long time to rise
- Worse fuel economy - a cold engine runs a richer mixture
- Check engine light on
What to check, in order
- Watch coolant temperature on the live data screen during a normal drive. If it never reaches roughly the normal operating range, the thermostat is the likely cause.
- Check the coolant level when the engine is cold. Low coolant can produce the same symptom.
- Feel the heater output. Poor cabin heat alongside this code strongly supports a thermostat stuck open.
- The thermostat is usually the cheaper part to replace and the more likely culprit, but confirm the sensor reads plausibly before replacing anything.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Fuel and air metering
P0100P0101P0102P0103P0104P0105P0106P0107P0108P0109P0110P0111P0112P0113P0114P0115P0116P0117P0118P0119P0120P0121P0122P0123P0124P0125P0126P0127P0128P0129P0130P0131P0132P0133P0134P0135P0136P0137P0138P0139P0140P0141P0142P0143P0144P0145P0146P0147P0148P0149P0150P0151P0152P0153P0154P0155P0156P0157P0158P0159P0160P0161P0162P0163P0164P0165P0166P0167P0168P0169P0170P0171P0172P0173P0174P0175P0176P0177P0178P0179P0180P0181P0182P0183P0184P0185P0186P0187P0188P0189P0190P0191P0192P0193P0194P0195P0196P0197P0198P0199