E7 error, system will not run in one mode (cool or heat) or at all

Safety: Shut off power to both indoor and outdoor units before inspection.

• Check thermostat mode: Ensure only one mode (cool or heat) is commanded. For Fraser‑Johnston communicating stats, verify no conflicting zone calls.
• Inspect reversing valve coil: At the outdoor unit, locate the reversing valve solenoid coil. Check for 24 VAC at the coil when calling for cooling (or heating, depending on system design). If voltage is present but the valve does not shift, the coil or valve may be stuck.
• Listen for valve shift: When switching between heat and cool, you should hear a noticeable change in refrigerant flow. No change suggests a stuck valve.
• Check wiring: Verify the O/B reversing valve wire is correctly connected at the thermostat, air handler, and condenser per Fraser‑Johnston wiring diagram.
• Persistent E7: If wiring and coil test good, the control board may be misinterpreting mode commands and may require replacement.