I have the same thing going on. P0128. Gonna try t-stat cause my temp gauge stays about 1/4... Leads me to believe t-stat is stuck open. I need to do a coolant system flush anyway. If that doesn't do it, I'll try the coolant temp sensor...

 

I learned the hard way when we first got ours replacing one thing after another and not fixing the problem and those parts add up after a little bit. could be Low engine coolant level leaking or stuck open thermostat faulty cooling fan (running too much) faulty coolant temperature (ECT) sensor, or a faulty intake air temperature (IAT) sensor
Yes, it very well could be the thermostat but it's best to start ruling out things first before just replacing things.

 

Well the coolant level and thermostat will be addressed with a system flush and fill which needs to be done cause I've got 80,000 on my truck and don't know when any service was done on it. The t-stat was $24. If the vehicle was warming up normally and kicking the CEL on, I'd be more inclined to think sensor but since it's not warming up properly, I'm thinking t-stat. If not, I've spent $24 on the piece of mind knowing I no longer have an 80,000 mile t-stat. The cooling fan clutch seems like a legitimate option but if the fan was running too much, it'd just be blowing on a radiator full of still coolant (since it hasn't warmed up enough to open a working t-stat). The coolant wouldn't flow till it got hot enough to open the thermostat and start cycling the coolant. If the t-stat was working correctly the coolant in the engine (which is where the coolant temp sensor is) would have an opportunity to warm up normally... But since the t-stat is open all the time, the coolant is always cycling through the radiator and never having a chance to warm up. The info I got from the obdII readout said low coolant, t-stat or coolant temp sensor. If I could test a thermostat without pulling it out of the engine I would. But since it's gotta be removed to do anything with it, I'll just replace it. I'm not getting any more codes or having any issues with the fan, I'll make a $24 bet and say t-stat.