Indoor unit display shows E0 and system will not start cooling or heating

Safety first: Turn off power to the Fraser‑Johnston air handler or wall unit at the circuit breaker and verify with a non-contact voltage tester before removing any panels.

Steps to try:

• Power reset: Turn the breaker OFF for 5–10 minutes, then restore power. Some E0 errors clear after a full power cycle.
• Inspect control wiring: Remove the indoor unit front cover. Locate the main control PCB and the display/keyboard PCB. Check all low-voltage harness plugs for full insertion, bent pins, or signs of corrosion. Firmly reseat each connector.
• Check for burn marks: Visually inspect the indoor main PCB for burnt components, swollen capacitors, or darkened areas. If present, the board has likely failed.
• Check line voltage: At the indoor unit’s power terminals, verify correct supply voltage (typically 208–230 VAC for Fraser‑Johnston split systems) with a multimeter. Incorrect or unstable voltage can corrupt the EEPROM.
• If E0 persists: The EEPROM or main PCB is likely defective. Replacement of the indoor main control board is usually required. Match the part number on the existing Fraser‑Johnston board and follow the manufacturer's wiring diagram when installing.

When to call a pro: If you are not comfortable working around live high voltage or handling control boards, contact a qualified HVAC technician. Incorrect wiring can damage the new board or compressor.