Fault Code Lookup

HVAC Sales Master
A100_O
Communications Lost with Indoor Unit
This code typically points to
  • Control/communication malfunction between indoor and outdoor units

Next checks to confirm
  1. Verify communicating bus and indoor-outdoor comm path
  2. Check daisy-chain topology and shield grounding
  3. Accessible snippet does not print full tree

Escalation
Lockout while active, short cycle to restart
Reset behavior
Clears when communication is restored.
Urgent
Fix now
Source
Rheem RP18AZ Installation Instructions Table 15