The pull pin was removed but the gauge still reads in the “green”- should I test the extinguisher to see if it works?
No, do not test. Carefully replace the pull pin where it should go and take it to a licensed fire extinguisher technician. The technician will check to see if the unit has been discharged. A unit that has been discharged (even a little) sometimes takes 2-3 days for it to lose the balance of its charge causing the gauge to show low pressure. An extinguisher with no “pressure” in the unit, will not work-have it serviced.