P0128: Thermostat: coolant not reaching normal operating temperature

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.

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 this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

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 the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• P0122 Throttle or pedal position sensor A: low signal
• P0123 Throttle or pedal position sensor A: high signal
• P0124 Throttle or pedal position sensor A: intermittent signal
• P0125 Engine takes too long to warm up for fuel mixture control
• P0130 Oxygen sensor (bank 1, sensor 1): circuit fault
• P0131 Oxygen sensor (bank 1, sensor 1): voltage too low
• P0132 Oxygen sensor (bank 1, sensor 1): voltage too high
• P0133 Oxygen sensor (bank 1, sensor 1): slow response
• 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

P0128 is a generic code, so it means the same on any make, for example on Alfa Romeo, Dacia, Kia, Nissan, Opel / Vauxhall, Škoda. See what Final OBD does on your car in makes and models.