P0128: Thermostat: coolant not reaching normal operating temperature
The coolant stays below the temperature the thermostat should hold it at. In the vast majority of cases the thermostat is stuck open.
| Code | P0128 |
|---|---|
| System | Engine and transmission |
| Group | P01xx, Air and fuel metering |
| Type | Generic: it means the same on every make |
| Can you keep driving? | Yes, no rush |
What P0128 means
The thermostat is a valve that keeps the engine at its working temperature by closing off the radiator while the engine is cold. This code means that after a period of driving the coolant is still colder than it should be, so the engine is being overcooled.
Most common causes
- Thermostat stuck open or not closing fully (the most common cause)
- Low coolant level
- Coolant temperature sensor reading lower than the real value
- Poor contact at the coolant sensor connector
- Radiator fan that never switches off
Symptoms
- Engine warning light on
- Temperature gauge stays low, especially on open roads or in cold weather
- The heater gives little warmth
- Slightly higher fuel consumption
Can you drive with P0128?
Yes, no rush You can drive normally; running a little cool will not harm the engine. It does use more fuel and wears more, so replace the thermostat before too long.
Open the bonnetthe in-depth explanation
How to check it
Watch the coolant temperature in live data after 15 to 20 minutes on the open road: if it drops noticeably while moving and rises when you stop, the thermostat is almost certainly stuck open. Compare it with the dashboard gauge too, to rule out the sensor.
How to read and clear P0128 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0128 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P0124 Throttle or pedal position sensor A: intermittent signal
- P0125 Engine takes too long to warm up for fuel mixture control
- P0126 The coolant temperature is too low for stable engine operation
- P0127 The intake air temperature is above normal
- P0129 The barometric pressure is below normal
- P012A The turbocharger intake pressure sensor has an electrical fault
- P012B The turbocharger intake pressure sensor gives readings outside the normal range
- P012C The turbocharger intake pressure sensor gives a lower signal than normal
- P0100 Mass or volume air flow sensor circuit fault
- P0101 Air flow sensor range or performance problem
- P0102 Air flow sensor circuit low input
- P0103 Air flow sensor circuit high input
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: Cooling.
What does code P0128 mean?
The thermostat is a valve that keeps the engine at its working temperature by closing off the radiator while the engine is cold. This code means that after a period of driving the coolant is still colder than it should be, so the engine is being overcooled.
Can you drive with P0128?
You can drive normally; running a little cool will not harm the engine. It does use more fuel and wears more, so replace the thermostat before too long.
How do you clear P0128?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0128 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
