How to change the coolant - 2003 GMC Yukon

Could it be a fuse?

A variety of sensors provide information to the ECM/PCM regarding engine operating characteristics. These sensors and their functions are described below. Be sure to take note that not every sensor described is used with every engine application. Coolant Temperature (ECT) The Engine Coolant Temperature Sensor (ECT) is a thermister (a resistor which changes value based on temperature) mounted on the engine coolant stream. As the temperature of the engine coolant changes, the resistance of the coolant sensor changes. Low coolant temperature produces a high resistance (100,000 ohms at 40°C/40°F), while high temperature causes low resistance (70 ohms at 130°C/266°F). The ECM/PCM supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ECM/PCM determines the engine coolant temperature. The voltage will be high when the engine is cold and low when the engine is hot. This information is used to control fuel management, IAC, spark timing, EGR, canister purge and other engine operating conditions. A failure in the coolant sensor circuit should either set a Code 14 or 15. These codes indicate a failure in the coolant temperature sensor circuit.
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It looks like the above, under hood, passenger side, upper engine area, mounted in front side of water outlet (according to Autozone.com).

without removing the head i don't see u getting them out
i don't understand how thermostat pumped coolant into intake manifold do u mean when you changed thermostat coolant splashed in throttle body if so that little amount of coolant wont matter now if some how cylinders got full of coolant just remove all spark plugs and crank motor over it will push all coolant out and what it didn't would end up in crankcase and oil change would take care of that
but un fortunately now only way to get tampoons out is to remove heads

how did you change the coolant? did you remove a hose or just open up the valve at the bottom of the radiator

If you removed the lower radiator hose then you may have induced air into the cooling system

when the engine is cool, remove the coolant cap and start the engine

wait until you have a good flow of coolant out of the reservoir to ensure you have all the air out of the system

reset the computer, if no change, replace the coolant sensor, but after reset it should work

The coolant sensor and the coolant level sensor are two different sensors. Your message center is showing (coolant sensor bad),not level sensor bad.
The level sensor only monitors the amount of coolant in the container
The coolant sensor senses the degrees of the coolant, which turns on the electric cooling fans. Change the coolant sensor, and the problem should go away.

Did you change you ECT (ENGINE COOLANT TEMPERATURE SCENSOR) ?

Which coolant sensor did you change? The one in the passenger side tank in the radiator.

If your oil is white looking on the dip stick, you will have serious damage inside the engine.