Ask Andy AIP0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
What it means, what causes it, and what it costs to fix.
What P0128 means
The engine is taking too long to warm up, or never reaches normal operating temperature. Nine times out of ten, the thermostat is stuck partly open.
Can I drive with P0128?
Severity: Low. Not urgent. Fix it when convenient.
Safe to drive, but the engine runs cooler than it should. That hurts fuel economy and can leave you with weak cabin heat in winter.
Get a diagnosis for your exact car
Tell Andy your year, make, model, and symptoms. You'll get best case, most likely, and worst case, with parts and labor costs. Free, in about 15 seconds.
Ask Andy about P0128Symptoms
- Heater blows lukewarm
- Temperature gauge stays low or takes a long time to rise
- Lower fuel economy
Common causes
Most likely first.
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
- Cooling fan running all the time
How to diagnose P0128
- Check the coolant level with the engine cold.
- Compare the scan tool's coolant temperature with an infrared thermometer reading at the thermostat housing.
- Make sure the cooling fan isn't stuck on.
Tip: most US auto parts stores will read your codes for free.
Repair costs
| Thermostat | $150–$400 installed |
| Coolant temp sensor | $80–$250 installed |
Typical US prices. Your vehicle, region, and shop rates will change these.
P0128 FAQ
What does P0128 mean?
P0128 means "Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)." The engine is taking too long to warm up, or never reaches normal operating temperature. Nine times out of ten, the thermostat is stuck partly open.
Can I drive with a P0128 code?
Safe to drive, but the engine runs cooler than it should. That hurts fuel economy and can leave you with weak cabin heat in winter.
What is the most common cause of P0128?
The most common cause is: Thermostat stuck open. Other causes include faulty coolant temperature sensor and low coolant level.
How much does it cost to fix P0128?
Typical repairs run about $80 to $400 in the US, depending on the cause. Thermostat: $150–$400 installed.
Still not sure what's wrong?
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