The coolant level sensor is in the radiator. Very often, when you fill the radiator and start the engine, the coolant level goes down. If you are still flushing the radiator, don't worry about this until you are done flushing it a few times.
Once you have cleaned the system out (get clear water) and filled the system with a mix of coolant and clean water, then you can check the level in the radiator when you first start the engine. What I usually do is do that final fill, with the coolant a little below the top of the radiator cap, and leave the radiator cap OFF. Then I start the engine and let the thermostat open (top radiator hose starts to get hot). At that time, the level in the radiator usually drops a little and you may be able to see the coolant "flowing" by the opening. At that time, I "top off" the level in the radiator (to the very top) and put the cap on. Then I fill the overflow bottle to the correct level, usually mid-way between Full Cold and Full Hot. That should take care of it.
Here is where the coolant level sensor should be (mounted on the radiator side tank - the square-looking thing). I would check that the connector has not come loose, if the level in the radiator is full.
As for this other question, you can either pay for it as a separate question (by clicking accept again) or consider it part of the other question. The choice is yours. I am happy I could help either way.
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Please let me know how this project works out or if you have more questions.
Take Care, Greg A.