Carel AL21

What does Carel AL21 mean?

AL21 is an Outlet Air Temperature Probe fault on the Carel Standard Air-conditioner (030221426). The temperature sensor monitoring the air supply temperature at the unit outlet has produced a reading outside the valid range, indicating a disconnected wire, failed sensor, or short circuit. The probe alarm triggers after the 60-second fixed delay and the controller may revert to fallback control.

Symptoms

• AL21 displayed on the controller after a 60-second alarm delay.
• Outlet air temperature reading shows an invalid value or dashes.
• Temperature-controlled functions using the outlet probe may use a fallback setpoint.
• Alarm relay activates per configured relay output assignment.

Common causes

• Outlet temperature probe wiring disconnected at the controller terminal or probe body.
• NTC probe element failed — reads open circuit (very high resistance) or short circuit (near zero).
• Probe damaged by excessive heat or moisture at the duct installation location.
• Incorrect probe type selected in configuration parameters.
• Wiring harness chafing causing intermittent short circuits.

Diagnostic steps

1. 01

   Inspect probe wiring at the controller terminal

   Check the signal wires at the terminal block for looseness or breaks. Tighten any loose connections. A disconnected wire is the most common cause of this alarm.

2. 02

   Measure probe resistance at the probe body

   Disconnect the probe and measure resistance across the sensor leads with a multimeter at ambient temperature. Compare to the NTC table for the probe type. An open or shorted probe must be replaced.

3. 03

   Inspect the probe installation in the duct

   Check the duct-mounted probe for physical damage from vibration, moisture intrusion, or heat exposure. Relocate the probe if it is in an abnormally hot or wet location.

4. 04

   Verify probe type configuration

   In the controller service menu, confirm the probe type parameter matches the installed NTC probe. A type mismatch causes valid resistance readings to appear out of range.

5. 05

   Replace the probe and confirm operation

   Install a compatible replacement probe. Confirm the outlet temperature reading is stable and plausible before clearing the alarm and returning to normal operation.

When to call a professional

Contact a qualified HVAC technician if replacing the outlet probe does not resolve AL21, which may indicate a faulty analog input on the pCO controller board. Board replacement and reconfiguration at the manufacturer level require Carel-authorized service personnel.