E6 is displayed and the indoor unit may run the fan but no cooling or heating occurs.

Safety first: Turn off power to both units before touching wiring.

Steps:

• Reseat communication wires: At both indoor and outdoor terminal blocks, loosen and re-tighten the communication terminal screws. Ensure copper conductors are fully inserted and not broken.
• Separate power and signal cables: If the communication cable is bundled tightly with high-voltage power cables, reroute it (if possible) to reduce interference, following Bosch installation guidelines.
• Check for water ingress: Inspect junction boxes and cable entries for signs of water or corrosion. Dry and reseal with appropriate grommets or sealant.

If E6 remains: The communication circuitry on either the indoor or outdoor PCB may be faulty. Board-level diagnosis and replacement should be done by a Bosch-authorized technician.