My pot was a 'pos' on the PM25 it was shorting out in places early on.
Do note that it is often quite difficult to measure resistance accurately in circuit. If the pot shows out of range over the whole travel then something is wrong either with the tester or the pot. From one leg to the other should measure the full value of the pot. From the middle leg to either end should vary from very low to nearly full value. If not you have a bad pot. You can probably find a workable replacement at any electronic parts shop.