Toyota Sequoia P0128: Coolant Thermostat – Below Temp

P0128 is a relatively common OBDII trouble code. It is triggered when your Toyota Sequoia‘s coolant temperature fails to reach the proper operating temperature.

Toyota Sequoia P0128

It’s a generic powertrain code, which means that regardless of who manufactured the vehicle, it’ll have the same meaning (1996+).


Sequoia P0128 Symptoms

There aren’t many symptoms with P0128, unless it gets cold. Here are the symptoms associated with this trouble code:

  • Lack of heat– The biggest symptom of the P0128 trouble code in the Toyota Sequoia is going to be lack of heat. When the thermostat doesn’t bring the engine all of the way up to the proper operating temp, it means that the water going through the heater core isn’t as hot. This leads to less/no heat.
  • Service Engine Soon– Other than the heater not working as good, the only other symptom of P0128 is typically the service engine soon light itself.


P0128 Causes: Toyota Sequoia

P0128 Toyota Sequoia

Fixing P0128 is usually a pretty easy thing to do, since 90% of the time it’s going to be the thermostat that is throwing the code. Here are the most common causes of P0128:


  • Thermostat Stuck Open– Far and away, the most typical cause for the P0128 code is going to be a thermostat that is stuck open. When a thermostat is stuck open, it allows coolant to flow through the engine at all times. When this happens, the engine is “overcooled” and never reaches the proper operating temperature. How to Replace a Thermostat (video). Sequoia Thermostat
  • Wiring– If the thermostat itself isn’t the problem, it’ll be the wiring/sensor associated with engine water temp detection, or the coolant temperature sensor itself.


Toyota Sequoia P0128 Diagnosis

A good way to tell if it’s a sensor issue or an issue with the thermostat is going to be whether or not the engine is running at its proper operating temperature or not. It can be really difficult to tell this in a vehicle that doesn’t have a temperature gauge.

A good way to at least get a good idea as to whether or not it’s the thermostat or the sensor is to start the car cold and let it idle for ten or so minutes. If you feel little to no heat from the heater, then it’s a pretty good indication that the thermostat is the problem, and not a wiring issue.

Also, a mechanic will typically check under the hood where the radiator hose meets the thermostat housing. If it is easy to pinch the hose, typically water will be flowing through it. If you pinch your hose make sure that you are aware of the belted accessories and the fan. The fan can come on at any time.



P0128 is not a serious threat to the immediate drivability of your Sequoia. It should be noted that most modern fuel injected engines to like to run at 200+ degrees. Letting them run too cool can problems after an extended period of time. Good luck figuring out what is going on with your Sequoia!