Q: Check Engine code P0128

asked by on

The Check Engine light is on with the code P0128 for the coolant thermostat (coolant temperature below thermostat regulating temperature).

The cooling system temperature is regulated by thermostat, and the cooling fans order to keep it at optimal temperature for the motor to operate. It does this by keeping the thermostat closed and the fans off until the coolant reaches around 195 to 210 degrees then opens the thermostat to cycle the coolant through the radiator while the cooling fans blow through the radiator cooling the coolant. If the thermostat is stuck open then coolant will never stay inside the motor long enough to heat up to the specified temperature. This would set a P0128. To verify that the thermostat has failed a proper inspection of the Check Engine Light should be called out by a qualified technician, like the one from YourMechanic.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Check Engine code P0128

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Hello, I have a 2006 mazdaspeed 6 with 100,000 miles and a "blown third piston" is it worth fixing?

It's too bad that it failed this early in the game, but I don't think there's any future in fixing it. Just remember that an engine is only one of many very expensive parts on a car and once you've...

Q: Q: What other cars could fit my engine?

Corolla 1.8L engines for model years 1998 through 2008 all used the same head casting and the same block casting. However, beginning in August 1999, and continuing through model year 2008, all of the 1.8L engines were equipped with variable...

Q: What temperature should your thermostat open up?

Hey there. Most vehicles run with a 195 degree thermostat. You can talk to your local dealer or parts shop should be able to tell you the specific one recommended for your vehicle and if any optional ones are listed/approved....

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...