There are two temp sensors that measure coolant temp; one to turn the cooling fan on and off, and another to tell the car’s computer what the engine temp is. Others, such as the one that tells the computer the temperature of the intake air, aren’t involved in your problem.
If it’s a simple P0128, the T-stat may simply be stuck open, but since you have no history on the car and you’ve suggested symptoms that point to air in the system (a possible leak), you definitely want to check the system out thoroughly. Loss of coolant can result in serious engine damage.
The PCM (power control module) is the car’s computer that controls the tranny. The ECM (engine control module) controls engine operation. Different makes use different acronyms, so these may vary.
Different colored coolants are DIFFERNT coolants that have different additives and work different ways to inhibit corrosion. Using the wrong kind can result in engine damage, so be sure your system contains the correct type. There are two fluids that I like to get directly from the dealer, coolant and tranny fluid. The cost of having either wrong can be serious.
You are correct in where the radiator is. The plastic covers are there to direct airflow through the radiator and such that the fan draws the air through properly. They’re called “shrouds” but “plastic covers” is fine.
Burping the system, getting the air out, consists of getting the fill hole to be the highest point in the system, generally by getting the front wheels up on ramps SAFELY!!! That’s followed by turning the heater on high to allow the coolant to flow through the heater core, filling the radiator, then running the engine some, filling it again, running it some, and filling it again, then filling the reservoir, putting the cap on, taking it for a drive, and checking the level after the drive.
Be advised that many cars, and I think the Cav is one, have two cooling systems in the radiator, one for the engine and one for the tranny fluid. Leakage CAN occur between the two, and that ain’t good. Fluid mixing can destroy both the engine AND the tranny. If you have mixing, and based on the color of your coolant you MAY, the tranny and the engine will need to be flushed out and refilled after the radiator is replaced.
This is a lot of information to digest. Either start with the T-stat, or perhaps save yourself a lot of money in the long run by having a radiator shop check the cooling system out and give you a good diagnosis, perhaps even repair it. The cost will be well invested.
You may have noticed that I never mentioned the tempo sensor. Assuming this and being wrong can be very costly. Start with the assumption that you have a cooling system problem rather than a sensor problem and you may be glad you did.