P0128: Coolant Temperature Below Thermostat Regulating Temperature
P0128 usually means the thermostat is stuck open, so the engine never reaches ideal operating temp.
Severity: low · System: Powertrain · Safe to drive: usually, short-term. Fix before winter — engine runs cold and heat doesn't work well.
Symptoms
- Slow warm-up
- Poor heater performance in winter
- Slight MPG loss
Common causes
- Stuck-open thermostat
- Faulty coolant temp sensor
Fair repair prices
| Thermostat replacement | $180–$450 |
|---|---|
| Coolant temp sensor | $100–$250 |
Ranges are US market averages (parts and labor). If your shop's quote is above the high end, ask for a line-item breakdown or run it through the invoice checker.
Other codes
- P0300 — Random / Multiple Cylinder Misfire Detected
- P0301 — Cylinder 1 Misfire Detected
- P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
- P0430 — Catalyst System Efficiency Below Threshold (Bank 2)
- P0171 — System Too Lean (Bank 1)
- P0174 — System Too Lean (Bank 2)
- P0442 — Evaporative Emission Control System Leak (Small)
- P0455 — Evaporative Emission Control System Leak (Large)