Shower plunger change over stuck. Can the plunger be rmoved and replaced or will the whole shower mixing valve need to be replaced?

An issue could cause the problem you're experiencing with your Aqualisa Quartz shower with the shower's thermostatic control valve. This valve regulates the water temperature and maintains a consistent temperature.
One common cause for a shower to become too hot is a malfunctioning thermostatic control valve. The valve may be stuck open, allowing hot water to flow through the shower even when the temperature is set to the lowest setting.
Another possible cause could be an issue with the shower's mixing valve, which is responsible for mixing the hot and cold water to achieve the desired temperature. If the mixing valve malfunctions, it could cause the shower to become too hot.
I recommend checking the thermostatic control valve and the mixing valve for any signs of damage or wear. If either of them is damaged, they will need to be replaced.
If the shower is still too hot even though you have set the temperature to the lowest, it might indicate that the thermostatic valve is not working correctly and needs to be replaced or repaired.
It's best to call a professional plumber or a service technician to check the shower and diagnose the problem; they will be able to find and fix the issue, ensuring that your shower will be safe and functional again.
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Possable there is some rubber in it . But it could be blocked with scale fApart from the hot water the valve out to clean it not much more i can say .I would look on u tube and see if it shows how to unblock the valve .

A shower head is one item a mixing valve is another item. Exactly what is it that you are trying to do?
A shower head just unscrews but it is wise to use plumber's tape so that it doesn't leak. You may need a plumber to install a mixing valve. You can Google "how-to install a mixing valve" to get an idea all of the work involved and whether or not you are capable.

You need to replace the mixing valve. It's not terribly hard to do, if you have some tools and, if the valve is not stuck in the fixture. Here's a video on how to replace it. Take note of how the tabs on the valve are positioned so you can put it in the same way it came out. If cold and hot water are reversed, you need to take valve back out and rotate 180 deg. and put back in:
http://www.youtube.com/watch?v=J-SNBNijfQc
Dave
This is assuming you have hot water elsewhere in the house and not ot the shower. Check to make sure you have hot water at other faucets.

If all the other faucetts in the house are ok, you probably need a mixing valve. They are pretty easy to change unless they are stuck in the valve but, usually with a little coaxing they come out fine. Pay attention to the tabs when you take it out so you can put back in the same way. If your hot and cold water are reversed, pull the valve otu and rotate 180 deg. and re-install. Here is a video:
http://www.youtube.com/watch?v=J-SNBNijfQc

Your mixing valve(faucet) has a bad valve and is allowing the hot water to bypass and leak thru shower. You should replace the bad mixing valve or the faucet to repair and stop the leak.

1. Is there a water saving feature/washer ?
2. Was there good pressure before ?
3. Is there an adjustment at the control valve for how much hot/cold water is being balanced ?
4. Any debris stuck in valve ?

it may be very simple--you have to get your head underneath the tub spout and look to see if there is a "set screw" on it---if there is ,you unscrew it, which releases the screw holding the spout in place on the pipe coming out of the wall---jimmy it a bit and the spout will come off---don't yank too hard---if there is no set screw, then all you do is turn the whole tub-spout counterclockwise carefully, and unthread it from the pipe in the wall..replace w/ a new tub-spout--anywhere from $5 to $500

The mixing valve where the hoses attach to is stuck at the plunger inside of valve.or the magnets burn out.that pulls the plungers in side of valve. If this may be problem, replace valve. This is called a mixing valve.

its the mixing slide in there its stuck u can take it apart at the handle it pulls out shake it if it doesnt move its bad. what happens is over time sediment gets in there and seazes it up. if you take it apart and its not stuck then its ur mixing valve on your boiler is bad good luck