The Honeywell 51309223-175 Low Level Mux Thermocouple FTA was originally used with Honeywell's PM/HPM products. It was reported that this part number suffered a high failure rate, which was attributed to defects in the mercury-wetted relays used with this FTA.

Honeywell withdrew this part number from distribution, and replaced it with the 51305890-175 Low Level Mux FTA, which uses solid-state relays in place of the problematic mercury-wetted relays used with this part number.