Ruud air conditioner outdoor unit will not start; thermostat calls for cooling but condenser fan and compressor are both off, display or board LED shows E0.

Safety first: Turn off power at the outdoor disconnect and the indoor air handler/furnace breaker before touching any wiring. Use a non-contact voltage tester to confirm power is off.

• Step 1 – Verify thermostat demand: Set the thermostat to COOL and lower the setpoint at least 3–5°F below room temperature. Confirm the indoor blower runs. If not, the issue may be with the indoor unit or thermostat power.
• Step 2 – Check low-voltage fuse: Open the indoor air handler or furnace panel. Locate the 3–5A blade fuse on the Ruud control board (often purple or red). Inspect and test with a multimeter. Replace with the same rating if blown. Restore power and recheck.
• Step 3 – Inspect communication/control wiring: At both indoor and outdoor control boards, inspect the low-voltage wires (typically R, C, Y, and communication terminals). Look for loose screws, corroded terminals, rodent damage, or splices that have pulled apart. Tighten all terminal screws and remake any damaged connections using proper wire nuts or crimp connectors.
• Step 4 – Check for 24 VAC at outdoor unit: With power restored and thermostat calling for cooling, measure between R and C at the outdoor control board. You should see ~24 VAC. If no voltage is present, the problem is upstream (indoor board, transformer, or wiring).
• Step 5 – Board inspection: If wiring and voltage are correct but E0 persists, inspect the outdoor control board for burn marks, swollen components, or water damage. If present, the board likely needs replacement.
• When to call a technician: If you are not comfortable working around live 24 VAC circuits or if both boards appear to be communicating incorrectly despite proper wiring and voltage, contact a qualified Ruud technician for board diagnostics and replacement.