Here are instruction to remove the filter housing. This is from my tips posting. Culligan filter housings are just Pentek/Pentair water filter housings and these tips apply to virtually any filter housing.
Frequently whole house filters can be very difficult to remove. The
canisters become almost impossible to loosen, even after pressing the
pressure relief valve and using a housing wrench. The most common
reasons for this are the housing was put back on with the housing
wrench, debris in the housing threads and a bad pressure relief valve.
How to remove a stuck filter housing.
1. Turn off the incoming water.
2. By-Pass the Softener
3. Go to the nearest cold water faucet, open it and let the water run until pressure drops significantly.
4. Turn the water off at outlet of filter (If housing has shutoff on top off head, turn this off now).
5. Press and hold the pressure relief button to relieve pressure further.
6. Use filter wrench to unscrew housing (If it still won't loosen repeat steps 2-5).
After removing, clean housing thoroughly, make sure threads are clean
on filter bowl and housing head. Lubricate threads of housing and head
with plumbers faucet lube or silicone.
8. Clean and lubricate the
O-Ring (replace the O-ring every couple years or if housing leaks after
screwing back on). It helps to flip the O-Ring over a couple times per
9. Replace the cartridge filter and screw the housing back on.
10. Turn the incoming water on slowly while holding the pressure relief button down until all air is purged.
11. Open outlet and let water run to cold water faucet until all air is purged.
12. Put softener back in service.
IMPORTANT! Only tighten the filter housing by HAND!
NEVER use the housing wrench to tighten the housing.
I hope these tips help you with the angst of removing stubborn filter housings.