Kia Soul: Bad Thermostat → Symptoms & Diagnosis

The thermostat in your Kia Soul’s engine plays a crucial role in regulating its temperature. A malfunctioning thermostat may lead to various issues and should be dealt with immediately.

Some of the most common symptoms of a bad thermostat are no heat, a spiked temp gauge, a blown head gasket, and more.

Kia Soul Bad Thermostat Signs

A thermostat failure can only happen in two ways.  

  1. The thermostat stays in the closed position and won’t let coolant through when the engine reaches operating temperature.  
  2. Or, the thermostat will stay open, and the coolant will flow through the engine continuously.

Thermostat Stuck Open Symptoms

Thermostat stuck open kia soul

There are a few indications that your Soul’s thermostat is faulty.  Hopefully, you’ve caught them before any damage was done to the engine.  Thermostats are very inexpensive if you do end up needing one.  

A stuck open thermostat can cause minor problems, including poor fuel economy and a lack of power, but it’s hardly noticeable.

Fortunately, if a thermostat is stuck open, there’s no risk of causing significant damage like blowing a head gasket or requiring other major repairs in your car.

Directly below are the typical indications of a thermostat that’s stuck open. 

P0128 Check Engine Light

P0128 is an OBD II trouble code indicating that your Kia Soul’s thermostat is stuck in the open position, meaning your engine isn’t getting up to the proper operating temperature. 

If your Soul has a P0128, it is almost certain that it needs a new thermostat.

No Heat

A bad thermostat will often result in no heat or less heat than usual coming from the heater core.  

When this happens, the heater will only work when driving in traffic or when you’re really pushing the engine. 

This cooling capacity is unnecessary on frigid days, and the engine never warms up.  Here’s more on no heat diagnosis if you are experiencing this.

Thermostat Stuck Closed

Below are the signs that your Soul’s thermostat is stuck open. If you suspect it is stuck open, don’t drive your truck. You can damage the engine if it gets too hot.


The most common symptom of a bad thermostat is overheating.  When the thermostat doesn’t open up, it keeps the coolant from entering the engine.  When this happens, the coolant in the radiator stays relatively cool, and the engine overheats fast.

Blown Head Gasket

The head gasket can blow if your Soul runs too long without coolant circulating through the engine.  Signs of a bad head gasket include:

  • Rough idle
  • Bad fuel mileage
  • Water in the oil
  • White smoke from the exhaust

A head gasket is a very costly and time-consuming repair.  Hopefully, you’ve caught your bad thermostat before this happens. 

Warning Light or Temp Gauge Spiked

Kia Soul Overheating Bad Thermostat

The first sign of a bad head gasket that most people will notice is a warning light or spiked temperature gauge.  

If you’ve noticed this, you must get off the road and get your Soul’s engine shut down as soon as possible before any lasting damage can occur.

A bad thermostat can cause the temp gauge or warning lights to come on.  But, it’s not the only reason.  Other things that can cause the temp gauge to spike are:

  • Low Coolant– If your Soul has been losing coolant, it’s possible that you’ve finally leaked out enough that the cooling system can no longer do its job.
  • Bad Radiator– If the radiator has clogged enough that coolant can no longer pass through efficiently, your Soul will overheat.
  • Water Pump–  The water pump is responsible for pushing coolant through the radiator and the engine.  Water can’t circulate through the cooling system when it goes bad, even if the thermostat is open.

Coolant Leak Around the Thermostat

As pressure builds around a bad thermostat that is stuck shut, you’ll notice that there may be coolant leaking around the thermostat housing.  

This leak is not the cause but rather a symptom of your Soul’s bad thermostat.

Conclusion:  Soul Bad Thermostat Symptoms

Replacing a thermostat is a relatively affordable repair.  If you have reason to believe it’s gone bad, ignoring it will only end up costing a lot more money in the long run. 

Good luck diagnosing your Kia Soul.  If there is anything that you would like to add, please leave a comment below.