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P0128 Coolant Thermostat (Coolant Temp Below Regulating Temperature)

What it means

The engine is taking too long to warm up to normal operating temperature, usually because the thermostat is stuck open.

Severity: Low to Moderate

Can I drive it? Drivable, but expect weak heat, lower fuel economy, and extra engine wear over time.

Common causes (most likely first)

Thermostat stuck open
Faulty coolant temperature sensor
Low coolant
Cooling fan running constantly

Likely fixes

Check coolant level
Replace thermostat and gasket
Test coolant temp sensor

DIY parts cost

$20–$80

Approximate estimate

Shop repair cost

$150–$400

Approximate estimate, parts + labor

DIY difficulty

Moderate

Varies by vehicle

Cost ranges are rough, approximate U.S. estimates and vary widely by vehicle, region, and parts brand. Always confirm the diagnosis before buying parts.

Parts you may need

ThermostatCheck price
Thermostat gasketCheck price
CoolantCheck price
Coolant temperature sensorCheck price

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