P0128 — Coolant Temperature Below Thermostat Regulating Temperature

P0128 means your engine isn't reaching its proper operating temperature. The #1 cause is a stuck-open thermostat that lets coolant flow before the engine warms up.

Cost range: $50–$400. Severity: low. Safe to drive? Yes, but expect worse fuel economy and excess engine wear in cold weather.

Most common causes of P0128

1. Stuck-open thermostat — Most common cause. Replace thermostat and gasket. ($50–$300)
2. Faulty coolant temp sensor — Sensor reports inaccurate low temp. ($30–$150)
3. Low coolant level — Check overflow tank first. ($10–$30)

Symptoms drivers report with P0128

• Heater takes long to warm
• Poor fuel economy in winter
• CEL