---
title: ALr 20 - How the 'Return Air Under Temperature' alarm works
description: If the cooler temperature has dropped below the normal mode setpoint by a specified amount, the SCS Controller can switch off the compressor and other outputs to protect the asset and trigger an AL20 alarm.
---
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July 2, 2026
# ALr 20 - How the 'Return Air Under Temperature' alarm works
## If the cooler temperature has dropped below the normal mode setpoint by a specified amount, the SCS Controller can switch off the compressor and other outputs to protect the asset and trigger an ALr 20 alarm.
Review the fault logic for the 'Return Air Under Temperature' alarm in the following graph:
### **Possible causes and actions**
Fault Reference
## ALr 20 · Return Air Under Temp
SCS 500 / 600 Controller
ALARM ACTIVE
WHAT TRIGGERS IT
If the cooler temperature drops **below the normal mode setpoint** by a set amount, the SCS Controller switches off the compressor and other outputs to protect the asset and raises **ALr 20** — the Return Air Under Temperature alarm. It typically shows up when the product keeps getting colder even after the compressor has turned off.
RESOLVE IT
CHECK
#### Verify compressor wiring & settings
Confirm the compressor is correctly wired, and that the **Compressor State** parameter is set correctly.
Still not resolved? It may be caused by an external secondary compressor relay failure.
Once wiring and settings check out, you're done.
Mark issue resolved
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