Carel AL027

What does Carel AL027 mean?

AL027 indicates a fault on the LT circuit inlet pressure probe (refrigerant side). The pressure transducer at the LT circuit evaporator inlet has produced a signal outside the valid range, which disables the circuit immediately. Without reliable pressure data, the controller cannot safely operate the LT refrigerant circuit.

Symptoms

• LT circuit 1 shuts down immediately and logs AL027.
• LT inlet pressure reading is invalid or stuck at a limit value on the display.
• Controller alarm relay activates based on configured alarm relay setting.
• Buzzer sounds and Alarm LED flashes on the user terminal.

Common causes

• Pressure transducer signal wiring loose or disconnected at the controller.
• Transducer failed due to overpressure event or manufacturing defect.
• Incorrect probe type or range configured in service parameters.
• Short circuit in the signal cable causing a 0 mA or 0 V reading.
• Actual refrigerant pressure at the probe location outside the transducer measurement range.

Diagnostic steps

1. 01

   Check probe connections at both ends

   Verify the connector at the transducer body and the terminal at the controller are tight and undamaged. Loose signal wires at either end create intermittent false alarms.

2. 02

   Measure transducer output signal

   With the system at known static pressure, measure the transducer analog output. On a 4–20 mA sensor, zero or non-proportional output at a known pressure confirms sensor failure.

3. 03

   Confirm configured pressure range

   In service programming, check that the probe range parameter matches the installed transducer. Installing a 0–10 bar transducer with a 0–30 bar range configured creates out-of-range alarms.

4. 04

   Replace the transducer if failed

   Recover refrigerant per applicable regulations before replacing the transducer. Install the correct pressure range replacement, leak-check the port, recharge, and restart.

When to call a professional

Refrigerant recovery and handling during transducer swap requires EPA 608 or equivalent certification. If the alarm persists after replacement, a qualified technician must evaluate actual refrigerant pressure levels and check the LT circuit for abnormal operating conditions that may be driving pressure outside the probe range.