I have a black plastic single pull out cartridge that is solidly stuck. I can't see any retaining clips or nuts. Am afraid to pull extra hard with pliers as the cartridge is plastic and am afraid I will break it. How can I get that thing out? Have tried twisting, turning, pulling.. Thanks.

We can't help until we know what we're dealing with. What type of faucet? What's it on? Do you have a model number? Let's have some details?:
Your cartridge is held in there by the "O" rings that have swelled and are now jamming the cartridge against the valve body. If you could get it to twist it would break the bond and make it easer to remove . If the cartridge has a threaded opening in the stem a Moen Cartridge Puller, (see image) might just be the answer, Good luck, Tom

Thank you. It is on the shower faucet. Pull to turn water on, turn left/right for water temp. Push in to shut off water.

The cartridge looks like Model # 988021. I can only see the black cylinder exposed. (The skinny long piece about 3 inches long, like a pencil shape) The body of the cartridge is located within the pvc pipe extending from inside the wall. The body of the cartridge, which I can't see, looks tight within the pvc tube. THe body of the cartridge has a circular piece,about the size of a quarter and this is the part that is within the pvc piping preventing me from determining if the cartridge is threaded or contains a clip/nut. Thanks again for all your help.

Cartridges don't thread in but they have either a clip or retaining nut to secure them. I don't see where you have much choice here but to first check for a clip and then put pressure on the cartridge to break it free. Try to twist it when you pull. Good luck, Tom

I got it out! The Moen puller worked instantly, the local Ace Hardware shop even let me borrow a used one, didn't have to buy it. It was a Price Pfistr Avante cartridge, no clip, no nut, was tight due to O rings swelling as you mentioned. Thanks for the help Speedball, am good to go.

Dontcha just love it when a plan comes together? Thanks for the input. I didn't really know if a Moen Cartridge Puller would work on a Price-Pfister Cartridge but I thought it might be worth a try. Cheers, Tom