Otis 211 is popping up STLD fault whenever unit is powered back up after power is off. Never attempts to make a run before it pops up. Does it no matter where the car is in the hoistway. 2 stop hydro. Fuses are good. Magnets are glued down to proper positions. Replaced the lithium battery on the main board. Slowdown and limit switches ohm out fine. Will do it even if it’s on inspection in the middle of the hoistway. No valve issue as problem only occurs upon powering unit back up. Am able to clear the fault and it will run fine after, until the next time power goes out. Any insight is appreciated. Leaning towards replacing one of either the main board or the selector board.

Maybe you already have, but I’d lean towards the selector board first. Besides, if you’re not 100% and you’re going roll the dice, the selector board is cheaper and easier to replace.
I’ve experienced the selector blipping twice in a step down from 3 to 2 causing a COR run. But after three recoveries, it’s down on on STLD (IIRC) hard fault. Your scenario is different, but….

Man, I don’t know… with that discrete selector you’re either getting inputs from it or you’re not. And to happen just sitting there on auto or even on inspect. I don’t even think you should be able to get an STLD on inspect. Sounds like something wrong with the IOBD. I just recently had a 211 getting a DLFB fault only on access or car top in the down direction. Auto fine, MR inspect fine, so weird. It was the IOBD but had to send to Unitec twice before they gave us a new one. I’d already swapped with another car to be sure I wasn’t missing something. If you want to rule out the selector, put it in inspect, disconnect it and cycle power. If it still happens you know the selector didn’t have anything to do with it.

I appreciate the comment. I’ll try disconnecting the selector next time I’m out and replacing the iobd if it fails to change the issue

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Got the main board repaired and that resolved. Thanks for the assist.