Carel AL015

What does Carel AL015 mean?

AL015 on the Carel FLSTDmSCHE screw chiller controller indicates the discharge temperature probe S4 on EVD Driver 1 for Circuit 1 is broken or disconnected. S4 monitors discharge gas temperature, which is critical for detecting superheated or overheating discharge conditions on the screw compressor. Loss of this signal causes an immediate stop of Compressor 1 — not a controlled shutdown — to prevent compressor damage from undetected high-discharge conditions.

Symptoms

• AL015 displayed on pCO5 terminal with Compressor 1 stopped immediately
• Discharge temperature reading on Circuit 1 absent or showing an implausible value
• Hard, immediate compressor stop rather than the staged controlled shutdown

Common causes

• S4 NTC discharge temperature probe has failed — open circuit or resistance beyond measurement range
• Wiring break on the S4 probe cable near the hot discharge line where thermal degradation is common
• Loose terminal screw at EVD Driver 1's S4 analogue input
• Probe physically dislodged from its mounting point on the discharge line

Diagnostic steps

1. 01

   Locate S4 discharge temperature probe on Circuit 1

   The S4 probe is typically clamped to the discharge line between the compressor outlet and the condenser inlet. Confirm its physical position and check that it is securely clamped to the pipe.

2. 02

   Inspect probe wiring for heat damage

   Power down the unit and inspect the S4 probe cable from the probe to the EVD Driver 1 S4 input. Discharge line temperatures can degrade cable insulation — look for brittle or cracked insulation near the probe tip.

3. 03

   Measure probe resistance

   Disconnect the S4 probe at the EVD driver terminal and measure resistance with a multimeter. Compare against the NTC resistance-temperature chart. Open circuit or zero resistance confirms probe failure.

4. 04

   Replace the S4 probe

   Install a new CAREL-compatible NTC discharge temperature probe. Use high-temperature rated cable if the original failed due to heat degradation. Secure the probe to the discharge line with a pipe clamp, not tape.

5. 05

   Reset alarm and verify immediate restart

   AL015 resets automatically once a valid S4 signal is detected. Confirm Compressor 1 restarts and discharge temperature reads plausibly. Monitor the first few minutes of operation for stability.

When to call a professional

AL015 triggers an immediate compressor stop (not controlled), which means the refrigerant circuit may have been in an abnormal state. After probe replacement, a licensed HVAC/R technician should verify discharge superheat and system pressures to confirm no damage occurred during the unprotected period. Probe work near hot discharge piping carries burn risk and requires proper protective equipment.