What It Means
- RETURN AIR TEMP SENSOR
Lennox Unit controller
Lennox Unit controller Error Code 78
Unit controller
Causes & actions from Lennox
How it shows
- M4 alarm code: 78
• If measurement of the return air temperature sensor is out of specified range (including open / short detection, the alarm will be activated. Alarm will automatically clear once in range condition is detected. • Check sensor and wiring.
Applicable models & manual sources
- MODEL L - M4 UNIT CONTROLLER
RETURN AIR TEMP SENSOR
MODEL L - M4 UNIT CONTROLLER, M4 Unit Controller
Disclaimer: This information is for general reference. Code wording and instructions vary by model. Always follow the service manual for your exact unit.
What should you check next?
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