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1001 / A21/A22/A23/A24

Alarm A21/A22/A23/A24 — No communication between system controller and remote control

Bosch · Alarm List A/W Split (8754) 2018 01 10

What does 1001 / A21/A22/A23/A24 mean?

The room sensor displays an error code (A21-A24) indicating no communication with the parent controller (HMC300/HPC400) via the EMS bus.

Common Causes

  • Poor EMS connection between display and installer module

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Check the EMS cabling at both ends, between the display and the installer module.

  2. 2

    Ensure secure connections.

  3. 3

    Contact a qualified technician if the issue persists.

Browse all codes in this manual (60)
Verified technical data. Last updated: April 2026

Related Faults

1010 / A11Alarm A11 — No communication via EMS BUS connection1037Alarm 1037 — Outdoor temperature sensor faulty, heating standby mode active1038Alarm 1038 — Invalid time/date
Source: Alarm List A/W Split (8754) 2018 01 10
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