Samsung split system / headpump in ceiling unit error

New here, hope this is the right place for this post!
thanks

Have a samsung split system that in a residential attic.

Samsung M # on indoor unit MH035FECA

The thermostat display unit has an error code E123
Error code is

Indoor unit heat exchanger out temperature sensor error (Open/Short)

Customer called, ordered part, replaced it (wasn't easy)
Start unit up and same error code.

I did notice on the display screen, that there was an A 20 code but looking on the manuals online couldn't find anything regarding A 20.

If anyone has any experience with this issue, help would be greatly appreciated!

Thank you!