Fault Code Lookup

HVAC Sales Master

21

Incompatible Equipment Detected

This code typically points to

Mismatch like single-stage non-variable-speed furnace with modulating outdoor unit

Next checks to confirm

Use compatible equipment

Escalation

Service urgent

Reset behavior

Clears when compatible system configuration is restored.

Urgent

Fix now

Need deeper detail?

Retrieve the subcode for this platform:

On thermostat: Menu > Settings > Advanced Settings > Dealer Control Center > Notifications
Review active alarms on homeowner notification screen or in Dealer Control Center > Notifications
For service-app access, press the smart hub commissioning button once
Connect the mobile device to the smart hub when the LED begins blinking green
Use the app as a service tool after system commissioning is already complete (S30 service session auto-logs out after 30 minutes)

Tool required: iComfort Mobile Setup application for service workflow

Source

Residential Communicating Systems Alert Code Guide, p.8

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