Warning: that $89 glow plug tester almost cost me a head gasket

I was chasing a cold start miss on a 7.3 and picked up one of those cheap inline glow plug testers off Amazon. It showed all 8 plugs were fine, but the truck still cranked like a dying battery. Turns out the tester was reading voltage through the bus bar, not each plug individually, so a bad plug on cylinder 6 slipped right past. I pulled the harness and tested each plug with a multimeter, found the dead one, and swapped it. Cost me two extra hours and a near misdiagnosis that would have had me pulling the intake for nothing. Has anyone else had a tool that gave them a false sense of security like that? I'm going back to the simple test light from now on.

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