Rheem mini‑split or inverter air conditioner shows E1 and stops cooling or heating; outdoor unit may not start.

Safety first: Turn OFF the breaker and outdoor disconnect before opening any panels. Confirm power is off.

• Verify wiring polarity: At both indoor and outdoor units, remove the electrical covers and compare the low-voltage terminal strip wiring (often labeled S1/S2/S3 or 1/2/3). Ensure each conductor goes to the same terminal number and polarity on both ends.
• Tighten terminals: Check that all terminal screws are tight. Loose screws can cause intermittent E1 communication errors. Retighten with an insulated screwdriver.
• Inspect cable condition: Follow the communication cable along its route. Look for cuts, rodent damage, UV cracking, or splices. Any damaged cable should be replaced with the correct gauge shielded cable recommended by Rheem.
• Check for shorts: With power off, use a multimeter to check continuity between conductors and to ground. Any short indicates damaged cable or terminals that must be repaired.
• Board diagnosis: If wiring is correct and intact, but E1 persists, either the indoor or outdoor control board communication circuit may be defective. A technician can swap-test or measure signal voltages to identify the failed board.

Warning: Never move or rewire communication terminals with power on. Incorrect wiring can destroy expensive Rheem inverter boards.