P0128 Code: Coolant Thermostat (Coolant Temperature Below Regulating Temperature)

🟠 Moderate severity · drivable

Safe to drive, but the engine running too cool hurts fuel economy and heater performance, so replace the thermostat soon.

What P0128 means

The engine is taking too long to reach (or isn't maintaining) its normal operating temperature. Usually this means the thermostat is stuck open, letting coolant circulate through the radiator constantly so the engine never fully warms up.

Symptoms

Check engine light is on
Engine temperature gauge reads low
Heater blows lukewarm air
Reduced fuel economy

Common causes

most common: Thermostat stuck open
common: Low coolant level
less common: Faulty engine coolant temperature (ECT) sensor

How to fix it

Fix	DIY?	Typical cost
Check and top up the coolant if it's low	Yes	$10–30
Replace the thermostat	Yes	$20–80
Replace a faulty coolant temperature sensor	Yes	$30–100

Frequently asked questions

Can I drive with a P0128 code?

Yes. The engine just runs cooler than normal. It's not an emergency, but fix it soon for better fuel economy and heat.

What is the most common cause of P0128?

A thermostat stuck open. It's a common, inexpensive part to replace.

How much does it cost to fix a P0128?

Usually $20–$80 for the thermostat itself, plus about an hour of labor if you have a shop do it.

Related codes

P0117

P0117 Code: Engine Coolant Temperature Circuit Low Input

P0118

P0118 Code: Engine Coolant Temperature Circuit High Input

P0125

P0125 Code: Insufficient Coolant Temperature for Closed Loop Fuel Control

P0128 by vehicle brand

Causes and repair costs for P0128 differ between makes. Pick your brand for the specifics.

P0128 on a Mercedes-Benz

Volkswagen

P0128 on a Volkswagen

Updated 2026-08. This page is informational and not a substitute for professional diagnosis.