1997 Ford Ranger DTC P0125

Hey guys, i'm trying to fix my uncle's pickup. So the only code that it's throwing "insufficient coolant temperature for closed loop fuel control" The temperature gauge reads below cold, but sometimes goes up to right on the cold line.

Here is the freeze frame data.

Fuel System 1 ================ Closed loop-using 02 feedback
Fuel system 2 ================ Not used
Calculated Load =============== 54.1%
Engine Coolant ================ 58 degrees Celsius
Bank 1 Short Term Fuel Trim ====== 3.1%
Bank 1 Long Term Fuel Trim ====== -2.3%
Bank 2 Long Term Fuel Trim ====== -3.9%
Engine RPM ================== 1800 RPM
Vehicle Speed ================ 16 km/h

So by my reasoning i don't think it's a bad ECT sensor, because the temp gauge does look like it's working, because it does go up a little bit. And since there's a ECT sensor and ECT sender (please correct me if i'm wrong) they couldn't both go bad at the same time. Unless they're on the same circuit... So i'm thinking it's most likely a stuck open thermostat. But even when i run the truck for a long time and drive it. It still doesn't reach normal operating temperature(according to the gauge). Is it possible that with a thermostat stuck open that it will never reach normal operating temperature? (also it's winter so keep in mind that it's 40-50 degrees Fahrenheit outside. I'm just trying to see what i'm up against before i tell my uncle i can fix it.
Thanks for your help.

Edit: Is there any way to test the ECT sensor without a scan tool? Like putting it in hot water and looking for voltage?

You already fixed it. The thermostat is probably stuck open or is low on coolant.

Okay, so i changed the thermostat and replaced the coolant with a 50/50 mix. And now the truck is running at operating temperature. Thanks for your help guy!