All codes › P0128
P0128: Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
What it means
Coolant stays below the thermostat's regulating temperature after a long warm-up. Almost always a thermostat stuck open.
Severity: Low Drivable; poor heat and economy in cold weather.
Common causes
- Thermostat stuck open
- Coolant temp sensor reading low
- Cooling fan running constantly
- Low coolant
Most common causes first. Exact causes vary by make, model and engine.
Can I fix it myself?
Often DIY. A thermostat swap is one of the most common DIY jobs; use the OEM temperature rating.
Repair cost
SEEDED ESTIMATE Roughly $130–$350
Most likely repair: thermostat. Diagnosis fees, taxes and other causes are not included.
| Line | Low | High | Basis |
|---|---|---|---|
| Thermostat | $31 | $31 | Typical listed retail range, source (single example listing ($31.49); coolant not included) |
| Labor 1–2 hr | $100 | $320 | $100–$160/hr, shop survey |
⚠️ This is an editorial placeholder range for this type of repair. It is not based on real invoices or licensed pricing data yet. Real ranges will appear here once enough reader invoices for P0128 are verified.
Verified reader invoices for P0128: 0. Share your repair bill to help build real costs.
Tools that can read P0128
- ScanFixAI: Pro $29.99/mo or $249.99/yr
- OBDAI: Annual PRO bundle $299
- BlueDriver Pro Next-Gen: $119.95 list ($99.95 sale seen)
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