Thermostat
Coolant Temperature Sensor
Intake Air Temperature Sensor
Coolant
Cooling Fan
Cooling Fan Relay
The necessary parts and tools to clear this Diagnostic Trouble Code (DTC) might differ depending on your vehicle's make and model. If your Check Engine light is illuminated, stop by any O’Reilly Auto Parts and get an O’Reilly VeriScan® for your car, truck, or SUV and get a detailed VeriScan report with more information about your vehicle’s Diagnostic Trouble Code(s), along with possible causes and repair solutions. If you aren’t comfortable making these repairs yourself, our parts professionals can recommend a local shop to make your repair.
OBD-II fault code P0128 means “Coolant Temperature is Below Thermostat Regulating Temperature”, and typically appears when your engine isn’t warming up to operating temperature as quickly as it should, or reaching operating temperature at all after you start your vehicle.
P0128 Code Diagnosis
When you experience a P0128 diagnostic trouble code, your vehicle may be experiencing failure in one of a few areas:
- Thermostat
- Coolant temperature sensor
- Intake air temperature sensor
- Low or contaminated coolant
- Defective cooling fan or cooling fan relay
One quick check that you can perform would be to inspect your cooling fan when the engine first starts. If you have electric cooling fans, they likely will not activate until the engine reaches operating temperature. If they’re running constantly, or even running when the engine is turned off, this could indicate a problem with the fans, fan relay, or coolant temperature sensor that could cause low cooling system temperatures and premature fan motor wear.
Often when this code presents, it is because the thermostat is stuck in the open position. This prevents the engine from circulating coolant until it reaches the appropriate operating temperature. You can check out our video on how to replace a thermostat in our How-To Hub.
P0128 OBD-II Code Symptoms
Symptoms of a P0128 Code can include:
- Check Engine light
- Temperature gauge reading low
- Reduced fuel economy
- Heater not working
If you’re comfortable diagnosing the issue yourself, or if you have other codes related to these symptoms, you may be able to determine the cause of your P0128 code with a physical inspection.
Whether you notice any symptoms or not, it is not recommended that you drive a vehicle with this code until the problem is diagnosed. Though low temperatures aren’t as critical as overheating, your engine is designed to operate most efficiently at a designated temperature, and failing to reach that temperature could affect performance.
Risks of Driving with a P0128 Code
Your vehicle may still run, but driving has the potential to make the problem worse or cause damage to other systems.
If your Check Engine light is on, stop by any O’Reilly Auto Parts location where our Parts Professionals can scan and diagnose your illuminated Check Engine light for free with our exclusive O’Reilly VeriScan code reading service. An O’Reilly VeriScan provides a detailed report of possible causes and solutions or repairs for your illuminated Check Engine light. The O'Reilly VeriScan report, along with the O’Reilly Auto Parts How-To resources can help you understand what parts, tools, and services you need to clear your DTCs and Check Engine lights. If you aren’t comfortable performing any necessary repairs yourself, we are also happy to recommend a local repair shop.
