Carel AL57

What does Carel AL57 mean?

AL57 signals that Driver 2's S2 probe (suction temperature NTC sensor) has failed or become disconnected. The S2 NTC probe provides the suction temperature reading that Driver 2 uses to calculate superheat. Without it, Driver 2 cannot compute superheat and shuts Circuit 2 off to prevent compressor damage.

Symptoms

• Circuit 2 goes offline immediately upon S2 probe failure.
• AL57 is displayed on the controller and requires manual reset.
• Driver 2 loses its superheat calculation capability.
• Compressors on Circuit 2 stop running.

Common causes

• Broken or disconnected NTC suction temperature probe wiring on Driver 2.
• Failed NTC temperature probe (S2) with open or short circuit.
• Probe installed in incorrect location away from the suction line.
• Corrosion or moisture damage at the S2 probe connector.
• Wiring routed too close to high-voltage cables causing interference.

Diagnostic steps

1. 01

   Check S2 probe resistance

   Disconnect the NTC probe from Driver 2 and measure resistance at the probe leads. At 25°C a standard NTC probe should read approximately 10 kΩ; an open circuit (infinite resistance) or short (near zero) confirms probe failure.

2. 02

   Inspect probe wiring and connectors

   Trace the S2 wiring from probe to Driver 2 input terminal. Look for breaks, pinching against sheet metal, or loose connector pins.

3. 03

   Verify correct probe installation on suction line

   Confirm the NTC is clamped tightly to the insulated suction line downstream of the evaporator and upstream of the expansion valve outlet, with proper insulation over the probe.

4. 04

   Replace the S2 NTC probe if defective

   Install a replacement Carel NTC probe (e.g., NTC006HP0R) and reroute the cable away from power wiring to avoid signal interference.

5. 05

   Manually reset AL57

   After restoring a valid S2 signal, manually reset the alarm from the controller menu. Circuit 2 will restart normal operation.

When to call a professional

If probe and wiring replacement does not clear AL57, the Driver 2 analog input circuit may be internally damaged. Driver board replacement and refrigerant circuit re-commissioning require certified refrigeration technician skills. Do not operate Circuit 2 without a functioning S2 probe — undetected liquid flooding will damage the compressor.