Carel AL51 Error Code

What does Carel AL51 mean?

AL51 activates when EVD Driver 2's electronic expansion valve failed to close completely during a power failure event. The Carel EVD evolution driver uses a backup power module (EVBAT) to close the EEV on blackout, preventing liquid refrigerant from flooding the evaporator when the compressor loses power. AL51 means this closing sequence did not complete successfully for Driver 2's valve.

Symptoms

• AL51 displayed on pCO terminal after power restoration
• EVD Driver 2 logs a valve not-closed event from the previous power failure
• Risk of liquid refrigerant in the suction line on next compressor start

Common causes

• EVBAT backup power module discharged or missing — insufficient energy to drive the valve to closed position during blackout
• EEV motor failure preventing valve movement during the emergency closing sequence
• Valve wiring fault that interrupted motor power during the blackout closing procedure
• Power failure was faster than the closing sequence could complete

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Diagnostic steps

1. Inspect the EVBAT backup module

   Check whether the EVBAT00400 or EVBAT00500 module is installed and properly connected to EVD Driver 2. The module must be fully charged before it can close the valve on power loss — check the charge indicator if present.

2. Verify valve motor wiring

   Inspect the 4-wire motor cable between EVD Driver 2 and the expansion valve for Driver 2's circuit. A wiring fault prevents the motor from responding to the emergency close command.

3. Test the emergency close function

   With the EVBAT module charged, simulate a controlled power interruption and verify the valve closes fully. The driver should complete the close sequence within the module's discharge time window.

4. Reset the alarm and check valve position

   Reset AL51 on the pCO terminal. Before restarting Compressor 2, confirm the expansion valve is at its pre-position setting and not stuck open, which could cause liquid slugging on start.

5. Replace the EVBAT module if depleted

   If the backup module fails to hold charge or cannot drive the valve to closed, replace the EVBAT module with a new CAREL unit. Allow the replacement to fully charge (approximately 4–5 minutes) before testing.

When to call a professional

If AL51 recurs after every power restoration event despite a functional EVBAT module, the expansion valve or EVD Driver 2 board requires diagnosis by a Carel-certified technician. Operating with a valve that cannot close on power failure risks compressor damage from liquid slugging on the next start sequence.