I have a shower faucet with a moen cartridge which has not been used for a while. It is stuck. I cannot pull it out to open the water valve to the shower. I have tried pouring boiling water on it and pulling.

I have a shower faucet with a moen cartridge which has not been used for a while. It is stuck. I cannot pull it out to open the water valve to the shower. I have tried pouring boiling water on it and pulling.
The reason the cartridge's stuck is because the "O" rings have swelled and are binding the cartridge. It will take a Moen Cartridge Puller,(see image) to remove it. You may purchase one at a plumbing supply house or rent one from your local rent center. Good luck, Tom

Agree with Tom. After you remove the old cartridge, and before you install new one, put lots of plumber's grease on O-rings and cartridge itself. Also, use plastic cartridge instead of brass.