Outdoor unit does not start, control board indicates E5, indoor blower may run normally

Safety first: Turn off power at the outdoor disconnect before opening the control panel.

Steps:

• 1. Locate ambient sensor: On Armstrong Air condensers, the ambient sensor is typically mounted near the coil or in the control compartment, with two small-gauge wires leading to the control board.
• 2. Inspect physically: Check that the sensor is not broken, crushed, or hanging loose. Look for UV-damaged or chewed insulation.
• 3. Check connections: Remove and reseat the sensor plug on the control board. Clean any corrosion with electrical contact cleaner and a soft brush.
• 4. Compare resistance: With power off, disconnect the sensor and measure its resistance with a multimeter. Compare to the Armstrong Air temperature/resistance chart (e.g., at 77°F many NTC sensors are ~10kΩ). If readings are far off, replace the sensor.
• 5. Reset system: Restore power and clear the fault by cycling power for 5 minutes. If E5 persists with a known-good sensor, the control board may be defective.