Outdoor unit will not run, thermostat calling for cooling, indoor blower may run but condenser is completely off

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.

Steps:

• 1. Verify thermostat demand: Set thermostat to COOL and lower the setpoint at least 3–5°F below room temperature. Confirm the indoor blower starts.
• 2. Check low-voltage fuse: Open the indoor unit’s control panel and locate the 3–5A blade fuse on the control board. If blown, replace with the same rating. If it blows again, there is a short in the low-voltage circuit.
• 3. Inspect communication cable: Trace the thermostat/communication cable from the indoor control board to the outdoor unit. Look for cuts, rodent damage, crushed sections, or loose connections at the terminal strip (typically R, C, Y, Y2, G, O/B, or proprietary comm terminals). Tighten any loose screws.
• 4. Test for 24 VAC at outdoor unit: With power restored, 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).
• 5. Reset control boards: Turn off power to both indoor and outdoor units for 5 minutes, then restore power. Some Armstrong Air systems will clear transient E0 faults after a full power reset.
• 6. If E0 persists: The outdoor or indoor control board may be defective. At this point, further diagnosis with the Armstrong Air service manual and a multimeter is required; contact a qualified HVAC technician.