Carel ALY04 Error Code

What does Carel ALY04 mean?

ALY04 on the Carel Smart HP ground-source heat-pump controller (program +030220741) indicates that the Smart HP has lost serial communication with the Clima zone terminal assigned to Room 02. The Clima terminals are small wall-mounted room controllers that communicate with the Smart HP master over a serial bus — they provide local room temperature control and display for each zone. ALY04 means the Clima for Room 02 is not responding to polls. The alarm has automatic reset and a 60-second detection delay.

Symptoms

• ALY04 displayed on the Smart HP pCO terminal.
• The Clima terminal assigned to Room 02 shows no data or a communication-lost message on its display.
• Zone 02 temperature control by the Smart HP falls back to defaults or last-known values.
• The alarm resets automatically once communication is restored.
• Alarm relay activation depends on parameter Gfc43 — it may not activate by default.

Common causes

• RS-485 serial bus cable to the Room 02 Clima terminal is broken, disconnected, or has high-resistance joints.
• Clima Room 02 has lost power — supply voltage absent at the terminal.
• Bus address conflict — another device on the pLAN or FieldBus has been assigned the same address as the Room 02 Clima.
• Incorrect bus termination causing signal reflections that prevent reliable communication to Room 02.
• The Clima terminal itself has failed — internal communication port damaged.

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Diagnostic steps

1. Confirm Clima Room 02 is unpowered or offline

   Physically locate the Clima terminal for Room 02. Check whether its display is blank. A blank display indicates power loss; a partial display or error message indicates the terminal is powered but not communicating.

2. Inspect the serial bus cable from Smart HP to Room 02 Clima

   Trace the RS-485 data cable from the Smart HP FieldBus or pLAN port to the Room 02 Clima terminal. Check both ends for loose screw terminals, broken conductors, and correct A/B polarity. Look for cable damage along the route.

3. Verify power supply to the Room 02 Clima

   Measure the supply voltage at the Clima terminal's power terminals. A Carel Clima is typically powered at 12–24 V AC or DC from the Smart HP or a local supply. Zero volts confirms a supply fault.

4. Check pLAN/FieldBus address of Room 02 Clima

   The Clima terminal address must match the address that the Smart HP expects for zone 02. Refer to the commissioning records and verify there is no conflict with another device on the bus at the same address.

5. Power-cycle the Clima and monitor for auto-reset

   After resolving any wiring, power, or address issues, power-cycle the Room 02 Clima terminal. Wait up to 60 seconds for ALY04 to clear automatically once communication is restored. Confirm Room 02 zone data reappears on the Smart HP overview screen.

When to call a professional

Contact a Carel-authorised controls technician or a building services engineer if ALY04 persists after checking all cable connections and addressing. Persistent communication failure on a single zone alongside functioning zones typically points to a cable fault or a failed Clima terminal requiring replacement. Replacing and recommissioning a Clima terminal on the Smart HP system requires access to the manufacturer parameters and pLAN address configuration.