Indoor unit shows E1 and the system will not start cooling or heating.

Safety first: Isolate power to both indoor and outdoor units at the breaker. Outdoor unit contains high-voltage components.

Steps to check:

• Verify wiring at terminal block: Remove the indoor unit front cover and outdoor unit service panel. Confirm that L, N, and communication terminals (often labeled 1, 2, 3 or similar) are wired identically at both ends. Tighten any loose screws.
• Inspect communication cable: Follow the multicore cable between indoor and outdoor units. Look for cuts, crushed insulation, or signs of rodent damage. Replace the cable if any damage is found, using the same gauge and type recommended by Bosch.
• Check for mixed polarity: Ensure live and neutral are not swapped and that the communication core is on the correct terminal as per the Bosch wiring diagram on the unit cover.
• Test continuity: With power off, use a multimeter to check continuity of each conductor between indoor and outdoor terminals. Any open circuit indicates a broken wire.

If E1 persists: The outdoor main PCB communication circuit may be defective and require replacement. This should be done by a qualified technician due to high-voltage and refrigerant components in the outdoor unit.