What It Means
- W4 STANDALONE
Lennox Unit controller
Lennox Error Code 521
Unit controller
Causes & actions from Lennox
How it shows
- M4 alarm code: 521
The W4 is unable to communicate to the M4 board.
Applicable models & manual sources
- MODEL L - M4 UNIT CONTROLLER
W4 STANDALONE
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?
Continue in ACLogics with guided diagnosis for your equipment and symptoms.
