title pretty much says it. finished soldering up my stack last night, this is the AM55 35x35 4in1 ESC, and when i power it up motor 3 just does this little twitch and then nothing. the other three spin up fine when i test them through betaflight motor tab. motor 3 just stutters and stops.

i reflowed the solder joints on that motor pad, looked cold to me, but same thing happening. the motor itself is fine i think, i swapped it with motor 2 and motor 2 started twitching in that position, so it's definitely the ESC pad or something with that output. joint looks shiny now but idk. continuity seems fine. running 4s if that matters. kind of at a loss here and dont wanna fry anything by just powering it up over and over

that swap test you did is actually the right move, confirms its the ESC output not the motor, good troubleshooting. a few things to check - first look really closely at the pad under magnification if you have it, bridged solder to an adjacent pad would cause exactly this. also check your motor signal wire connection if youre running individual signal wires, a bad connection there on M3 specifically would cause the stuttering you're describing.

if everything looks clean electrically it might just be a dead FET on that ESC output which is unfortunately pretty common with cheap 4in1s and theres not really a fix for that at home, you'd be looking at warranty or replacement. what does betaflight show in the ESC telemetry for that motor if you have telemetry enabled? sometimes it throws an error code that points you in the right direction.

had almost the exact same thing happen, turned out i had a tiny solder bridge between the motor C pad and a ground point nearby. couldnt see it with the naked eye, needed a loupe. worth checking before you write off the ESC entirely