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P0128 — Coolant Thermostat Below Regulating Temperature

Engine is not reaching operating temperature in the expected time. Almost always a stuck-open thermostat.

Severity: LowRepair: $15-$200Difficulty: Easy

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Common symptoms of P0128

Common causes of P0128

Repair cost & difficulty

Typical cost to fix P0128: $15-$200 · Difficulty: Easy

Pro tip: $15-50 thermostat replacement fixes this. A cold-running engine wastes fuel and wears faster.

Vehicles commonly affected

All makes — one of the most common DTCs

Related trouble codes

P0125P0115P0116

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ScanFixAI is a diagnostic aid, not a substitute for professional repair. Cost estimates are typical ranges and vary by vehicle and region.