My car has been throwing the P0128 code for the past 2 weeks. I was wondering if there was anyway to distinguish if the thermostat has gone bad or if the sensor has gone bad? The radiator and reservoir are full of coolant. I've noticed that after I have been driving the car for 4-5 minutes, the temperature goes up to about 150 degrees and then stays there; regardless if I'm driving for 5 miles or 80 miles.

 

as far as i know i don't know if there is a way to distinguish, you could replace the sensor which is easier to get to and is cheap (~11 bucks) first and see what happens

 

An infrared thermometer can be used to read the temperature of the lower intake manifold at the thermostat housing.

Beyond that, we need to know what you are driving to determine if there may be other ways to troubleshoot the problem. For instance, the 96-97 engines had a gauge temperature sending unit and a PCM temperature sensor. If both are reporting low temps, then the thermostat is likely at fault.

 

Its a 2001 Blazer 2wd.

 

P0128 - Engine coolant fails to reach preset target temperature

The thermostat is cheap and easy to replace. Due to their fail-safe design, when they do fail, it is in the slightly open position which will not regulate the temperature properly. If you cannot remember ever doing this repair, I would recommend that as a first course of action. Replace the t-stat with a Stant fail-safe model and also replace the radiator cap with a new Stant safety cap.

If the condition persists, you may have to change the temperature sensor, but the fact that the temperature gauge does move from cold up to ~150F means that the sensor is reading something.

 

The thermostat has been replaced. about 4-5 years ago, 30k mileage ago

 

That's a long time ago. And even if it was much more recent, new parts can be bad right out of the box.

 

when its ~150*F does it fluctuate, or fluctuate with the RPMs? or does it stay there?

 

It just sits at 150 the entire time, regardless of rpms

 

Depending on where you live and the current outside temperatures, 150F is very possible to maintain if the t-stat were stuck open. And, as I said earlier, since the gauge is moving, it is at least reading something.