Code C7 = Combustion fan fault. Control board commanded the fan to start and either didn't receive expected RPM feedback or fan didn't respond.
Diagnostic sequence
1. Listen for the fan
When the unit attempts to fire, you should hear the fan ramp up. If silent — fan motor or PCB output failure.
2. Inspect air inlet
Fan loaded by intake blockage. Clean intake filter and inspect vent termination.
3. Wiring
Harness from PCB to fan motor — loose or damaged connector.
4. RPM feedback sensor
Hall-effect sensor on the fan shaft. Failure means fan spins but PCB throws C7.
5. Fan motor
If motor is bad, replacement is dealer-level. $200-400 parts.
Bottom line
C7 is hardware. Re-seating connectors fixes some cases; persistent C7 = installer call.