Safety first: Turn OFF all breakers before removing access panels. Use a multimeter only if you are trained to work with live circuits.

Step 1 – Check breakers: Verify the indoor air handler/furnace breaker and outdoor condenser breaker are ON. Reset if tripped once; repeated tripping indicates a fault.
Step 2 – Inspect low-voltage fuse: On the indoor control board, locate the 3–5A blade fuse protecting the 24V circuit. If blown, replace with the same rating. If it blows again, there is a short in the thermostat or control wiring.
Step 3 – Transformer check: Listen for a slight hum from the 24V transformer. If there is no 24V between R and C at the board (tested by a qualified person), the transformer may be failed.
Step 4 – Wiring inspection: Look for loose or burnt wires at the line-voltage terminal block and disconnects.
Call a professional: Persistent H0 or repeated fuse blowing requires a technician to locate shorts and verify safe operation.

H0 code, unit does not start, display may be blank or show only H0

Possible Causes