Carel AL54

What does Carel AL54 mean?

AL54 means Driver 1's pressure probe S1 has failed or lost its signal. The probe S1 is the ratiometric pressure transducer that the expansion valve driver uses to calculate evaporation pressure and superheat. Without a valid S1 reading, Driver 1 cannot control the expansion valve and shuts Circuit 1 off.

Symptoms

• Circuit 1 shuts down immediately upon probe S1 failure.
• Controller displays AL54 with no automatic recovery.
• Superheat calculation for Driver 1 becomes unavailable.
• EEV may close to a safe default position.

Common causes

• Damaged or disconnected wiring between the S1 pressure probe and Driver 1.
• Failed ratiometric pressure transducer (S1 probe) on Circuit 1.
• Incorrect probe type configured in driver parameters (probe model mismatch).
• Moisture ingress or corrosion at the probe connector.
• Short circuit on the S1 signal wire.

Diagnostic steps

1. 01

   Verify S1 probe wiring connections

   Inspect the cable between the ratiometric pressure transducer and Driver 1's S1 input terminals. Look for loose connectors, pinched wires, or corrosion.

2. 02

   Measure probe supply and signal voltage

   With power on, measure the 5 V supply to the probe and the signal output (should be 0.5–4.5 V across the operating pressure range). A flat 0 V or 5 V signal indicates a failed transducer or open circuit.

3. 03

   Check the probe model parameter in Driver 1

   Navigate to Driver 1 parameters and confirm the configured pressure probe type matches the physically installed transducer's range and output type.

4. 04

   Substitute the pressure probe

   Replace S1 with a known-good Carel ratiometric pressure probe (e.g., SPKT0013P0 for -1 to 9.3 barg range). Re-enable Driver 1 and confirm the signal is valid.

5. 05

   Manually reset AL54

   After correcting the probe fault, reset the alarm from the pCO controller alarm menu. AL54 requires manual reset.

When to call a professional

If wiring and probe replacement do not resolve AL54, the Driver 1 input circuit on the expansion valve driver board may be damaged. Board-level diagnosis and replacement requires refrigeration technician expertise. Do not attempt to bypass the S1 probe signal — this disables a critical safety input and voids the manufacturer's warranty.