Indoor unit stops and displays E1, outdoor unit does not start

Safety first: Turn off the main AC breaker to both indoor and outdoor units before touching any wiring.

Steps:

• Verify power and polarity: At the terminal block (usually labeled S1, S2, S3 or 1, 2, 3), confirm that the same wires go to the same terminals on both indoor and outdoor units. Mitsubishi Electric systems are polarity-sensitive; reversed S2/S3 can cause E1.
• Check for loose connections: Tighten all terminal screws on indoor and outdoor units. Look for discolored or overheated terminals and repair/replace as needed.
• Inspect communication cable: Follow the interconnecting cable between indoor and outdoor units. Look for cuts, crushed sections, rodent damage, or splices. Repair with proper junction boxes and connectors or replace the cable if damaged.
• Measure voltage: With power ON (only if you are trained and have a meter), verify correct voltage between S1–S2 and S1–S3 as per the unit’s rating plate. Incorrect supply can cause communication errors.
• Board diagnosis: If wiring and voltage are correct and E1 persists, either the indoor or outdoor PCB communication circuit may be faulty. Swap-testing or board-level diagnosis should be done by a qualified technician.

When to call a technician: If you are not experienced with live electrical measurements or the wiring appears damaged inside the unit, contact a licensed HVAC technician.