Carel AL75

What does Carel AL75 mean?

AL75 indicates that the S2 probe on EVD Driver 2 — the suction temperature sensor for Compressor Circuit 2 — is faulty or disconnected. The Carel Standard Air-Conditioners program uses this probe to calculate superheat and protect the compressor from liquid slugging. On AL75, the controller shuts down Compressor 2 in a controlled sequence to prevent damage.

Symptoms

• AL75 displayed on pCO controller with Compressor 2 shut down in controlled mode
• S2 suction temperature reading absent or displaying an out-of-range value
• Circuit 2 cooling capacity lost; Circuit 1 may continue operating normally

Common causes

• Loose or broken wiring at the NTC suction temperature probe connected to EVD Driver 2 input S2
• NTC probe failure — resistance drifted outside the controller's valid measurement range
• Connector corrosion at the EVD Driver 2 terminal block input for S2
• Incorrect probe type selected in EVD Driver 2 parameters

Diagnostic steps

1. 01

   Inspect S2 probe wiring on Driver 2

   Shut down the unit and trace the S2 cable from the suction line NTC probe back to EVD Driver 2. Check for pinched insulation, loose terminal screws, or visible wire breaks.

2. 02

   Measure NTC probe resistance

   Disconnect the S2 probe wires at the driver terminal and measure resistance across them with a multimeter. At 25°C, a standard CAREL NTC should read approximately 10 kΩ. Readings of open-circuit or near-zero indicate probe failure.

3. 03

   Re-seat connectors and retest

   Clean the connector contacts, reseat the S2 wires, and restore power. If AL75 clears and suction temperature reads plausibly, the fault was a loose connection.

4. 04

   Verify probe type configuration

   Access EVD Driver 2 service parameters and confirm the S2 probe type is set to match the installed sensor. Incorrect configuration (e.g., NTC set to ratiometric) will generate a false fault.

5. 05

   Replace the NTC probe if defective

   If resistance measurement confirms probe failure, replace with a CAREL-compatible NTC suction temperature probe. After replacement, clear the alarm and verify Driver 2 restarts Compressor 2 normally.

When to call a professional

If probe replacement does not resolve AL75, or if Compressor 2 shuts down again shortly after clearing the alarm, have a certified technician inspect the EVD Driver 2 board and verify the refrigerant circuit superheat values are within specification. Repeated probe failures on the suction side can also indicate liquid refrigerant flooding the sensor location, which requires circuit diagnosis.