Originally Posted by 5picker
If it is a factory installed filter...
Try pushing upwards (towards the mounting head) on the canister before turning.
Most folks don't realize the head has teeth and the canister has tabs.
Pushing up on the canister releases the contact of the teeth/tabs so it can be loosened.
Conversely, when installing, twist until aligned and pull down on the canister slightly to lock the teeth/tabs in place.
A small dab of silicone grease around the o-ring seal makes the process a whole lot easier.
X2. Release the water pressure first, then follow the above instructions.
Not easy, but it does work.