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Not lit EXT COM (green)

LED: EXT COM (green) Not lit — No communication with indoor module/control module

NIBE · S2125 Air/Water Heat Pump

What does Not lit EXT COM (green) mean?

The green EXT COM LED on the base board (AA2) is not lit, indicating a lack of communication with the indoor module or control module.

Common Causes

  • Communication cable is not correctly connected or is faulty.
  • Indoor module/control module is not powered or functioning.
  • S2125 is not correctly installed in the indoor module or control module.

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Check that S2125 is correctly installed in the indoor module or control module.

  2. 2

    Inspect the communication cable connected to terminal block X22:1–4 for damage or loose connections.

  3. 3

    Ensure the indoor module/control module itself is powered on and functioning.

  4. 4

    Consult the Installer Manual for the indoor module/control module for specific connection details.

Browse all codes in this manual (55)
Verified technical data. Last updated: April 2026

Related Faults

Communication fault to Inverter 254Alarm 254 (or 264, 203) Communication fault to Inverter — Inverter unpowered or brokenCompressor speed too low 354Alarm 354 (or 503, 186) Compressor speed too low — Inverter safety function reduces compressor speedContinuous light CPULED: CPU Continuous light — CPU is not running correctly
Source: NIBE S2125 Air/Water Heat Pump
Open Manual (Page 32)Search more codes in this manual