EVCO AL13

What does EVCO AL13 mean?

AL13 on the Evco SAVE AHU controller (144SAVEU0E01) is a return air temperature probe fault alarm. The controller has detected that the return air temperature sensor is open, shorted, or reading outside its valid range. With AL13 active, the return air temperature control loop is inhibited — any control logic that uses return air temperature for zone temperature feedback or reset schedules is disabled until the probe is repaired. The alarm resets automatically once a valid sensor reading is restored.

Symptoms

• AL13 displayed on the SAVE AHU controller.
• Return air temperature display shows an error or dashes.
• Zone temperature control using return air feedback is inactive.
• Heating or cooling may operate at a fixed output rather than following return air demand.
• Alarm self-clears when a valid return air temperature reading is detected.

Common causes

• Return air sensor wiring has become disconnected at the controller terminal block.
• Sensor cable has been damaged — cut, pinched, or abraded — in the return air duct or plenum.
• Return air sensor has failed due to moisture exposure or age.
• Wrong sensor type installed relative to the controller parameter setting.
• Sensor is located in a dead-air pocket or near a bypass opening, causing erratic readings outside valid range.

Diagnostic steps

1. 01

   Inspect the return air sensor and its wiring at the controller

   Confirm sensor wires are seated fully in the controller terminal block. Tug each wire gently to verify it is clamped — a wire that pulls free without force was never properly terminated.

2. 02

   Measure return air sensor resistance

   Disconnect the sensor at the controller and measure resistance with a multimeter at the known return air temperature. Compare to the expected resistance curve for the installed sensor type. An open circuit indicates sensor failure.

3. 03

   Inspect sensor cable routing through the return air duct

   Trace the return air sensor cable from the sensing head to the controller. Look for damage at duct penetrations, flex-duct connections, or anywhere the cable passes through a structural member.

4. 04

   Verify sensor position in the return air stream

   Confirm the sensor is positioned in a representative section of the return air duct where it reads mixed return air temperature, not localized hot or cold spots from nearby diffusers or bypass openings.

5. 05

   Replace sensor and confirm alarm clears

   Install a replacement return air sensor of the correct type and resistance class. Reconnect at the controller, restore power, and confirm the temperature displays correctly and AL13 clears automatically.

When to call a professional

Engage a licensed HVAC technician if AL13 is accompanied by poor zone temperature control or if the return air sensor location needs to be repositioned for accurate readings — sensor placement requires knowledge of the duct system airflow characteristics. In critical environments such as hospitals, data centres, or cleanrooms, return air temperature sensor failure requires immediate technician response to prevent zone condition drift.