I can tell you from personal experience that I've had dealers deny warranty claims because my customer had an independent working on their machine (me). It's happened a few times with different dealers (different makes & models). They found an excuse about how the machine wasn't repaired to factory standards because aftermarket parts were used (filters). Now, my customer certainly could call the manufacturer as you stated, but small- one forklift shop owners wear many hats & getting into a **** contest on warranty is time consuming & puts a bad taste in everyones mouth.

This is why I always advise my customer to have the dealer do all the PM's on the unit during the warranty period. Not only is there no question on warranty work, but also any updates/ recalls, etc can also be handled.