I have a 2004 Lariat that’s showing a P0128 code, which means “engine coolant below regulating threshold.” I’ve already replaced the thermostat twice with a 190-degree one from the dealership, but that didn’t solve the problem. I also replaced the engine coolant temperature (ECT) sensor, but that didn’t work either. I even replaced the fan, but the code still comes up.
When I’m driving on the highway, the temperature gauge goes up to about a quarter of the way. In town, it stays in the middle, where it’s supposed to be. But as soon as I get back on the highway, the gauge drops back down to a quarter. Has anyone else had this happen? I’m really frustrated and would appreciate any suggestions.
It sounds like your issue could be related to a stuck-open thermostat or a radiator that’s overcooling at highway speeds. Even though you replaced the thermostat, it might still be defective. You could try blocking part of the radiator to see if the temperature stabilizes. Also, check for any coolant flow issues.
It sounds like you’ve covered most of the basics. Given the recurring P0128 code, I’d check for issues like a faulty thermostat housing, an air pocket in the coolant system, or even potential wiring problems affecting the ECT sensor. You might also want to verify that the replacement thermostat is functioning correctly and compatible with your vehicle.
The P0128 code on a Ford indicates that the engine coolant temperature is below the thermostat’s regulating temperature, often due to a faulty thermostat or low coolant level.