Not an emergency but fix soon. A stuck-open thermostat wastes fuel and the engine runs less efficiently. In cold weather your heater will blow cold air.

How much does a thermostat cost?

Thermostats cost $10-$25 for the part. With gasket and coolant, total DIY cost is $15-$40. Most take 30-60 minutes to replace.

P0128

Medium Severity

Coolant Thermostat Below Regulating Temperature

Cooling System

What Does P0128 Mean?

The engine is not reaching normal operating temperature within the expected time. The thermostat is almost certainly stuck open.

Common Fix

Replace thermostat

Estimated Cost (DIY)

$15 - $40

Symptoms

⚠️Temperature gauge stays below normal
⚠️Poor heater output
⚠️Increased fuel consumption
⚠️Slow warm-up

Common Causes

Stuck-open thermostatlikely

Faulty ECT sensorpossible

Missing thermostatpossible

Thermostat installed backwardsunlikely

How to Diagnose P0128

1Monitor coolant temp during warm-up
2Feel upper radiator hose — should be cold until thermostat opens at ~195°F
3Check if someone removed the thermostat
4Replace thermostat

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Related Codes

P0125 P0116 P0115

Frequently Asked Questions

Is P0128 serious?: Not an emergency but fix soon. A stuck-open thermostat wastes fuel and the engine runs less efficiently. In cold weather your heater will blow cold air.
How much does a thermostat cost?: Thermostats cost $10-$25 for the part. With gasket and coolant, total DIY cost is $15-$40. Most take 30-60 minutes to replace.

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