Sep 19, 2023 - 01:25 PM
The problem already existed even before the vehicle was tested for smog. Still, the code P0128 stands for Engine Coolant Temperature Below Thermostat Regulating Temperature. It means that the ECM/PCM has “noticed” that the engine coolant temperature is consistently running below the thermostat regulating temperature. In other words, the engine is running cooler than it should. A healthy engine should run above 200 degrees. The radiator cooling fan on some vehicles doesn’t even engage until after the coolant reaches nearly 230 degrees. one of the top possible causes is a low coolant level (an air pocket trapped at the engine coolant sensor). The 2nd possible cause is the engine coolant thermostat is stuck in an open position or is opening too soon. If the thermostat is stuck WIDE open rather than just barely open, a P0125 will be stored. Click this link for more info from our CarParts.com resource blog. I hope this helps!