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

Internal combustion engines tend to get rather hot as they’re in operation. Coolant is used to keep the engine temperature down so that overheating does not occur. Without enough coolant, the extremely hot temperatures could damage the internal components of the engine. Then you’d need to pay thousands of dollars to replace the engine.

When you start a car engine, it takes a certain amount of time for it to reach its appropriate operating temperature. Coolant cannot immediately start cooling down the engine before it’s had a chance to warm up. If the engine is exposed to coolant prematurely, then it won’t be able to start up successfully. That is why a coolant thermostat is used to prevent coolant from reaching the engine until it’s had enough time to warm up to the appropriate temperature.

The powertrain control module monitors the length of time it takes for this to happen. Once the proper operating temperature exists, the coolant thermostat is activated to regulate the temperature appropriately. It’ll allow just enough coolant through to ensure the engine does not overheat.

After you start your car engine, it should not take more than 15 minutes for the engine temperature to exceed 160ºF. While it is not good for your engine to overheat, you also never want it to drop below 160ºF either. That could also hinder its performance and make it more difficult for you to accelerate the vehicle. If the module detects the temperature has fallen under 160ºF, then it will produce diagnostic trouble code P0128.

Of course, you need to run a diagnostic scan in order to see this trouble code for yourself. You won’t have the good sense to run the scan until you notice certain symptoms associated with low engine temperature. These symptoms include a bad fuel economy, high idle, low-temperature gauge, and a Check Engine warning light activated.  When you experience any of these, run the diagnostic scanning tool on the powertrain control module to see what’s going on.

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When you see P0128 as the diagnostic code, it usually means your coolant thermostat is stuck in an open position. That would allow coolant to enter the engine when it is not supposed to be there. Sometimes the code could also mean you have a bad coolant temperature sensor or a radiator fan which doesn’t stop running.

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