I used to replace them every year or two,thinking i'll catch it before it fails.
I've since been waiting until they fail,but it's been 3-4 years,and they are still going strong.
I make sure they are mounted in an easy spot to get to,and carry spares,so I can swap them in a min or two if they ever fail.I do clean and grease the connections every year,so they will come off easy if I do need to change it.
If you can not change in field! (snowing,wind blowing 30+, 2am) You can jump it out to raise plow and get to better working conditions! Been there Done that. I keep a screw driver for jumper! Have someone hold up & short across leads!
I have a used Fisher EHP. I just finished mounting my solenoid
under the hood. It works fine, but I was wondering what size fuse to use in the "signal" side. The guy I bought it from just had the 14 or 16ga wire to the BAtt side of Solenoid, the manual from fishers site shows a fuse on that wire so I got a fuse holder but found no mention of the size fuse.