Lennox communicating thermostat reports E250 outdoor unit lockout; system will not cool until power is cycled or lockout is cleared.

Safety: Turn off power at the outdoor disconnect and indoor breaker before opening panels.

• 1. Power cycle to clear lockout: Turn off power to the outdoor unit for at least 5 minutes, then restore. Some Lennox controls require a thermostat reset as well. This may temporarily clear the lockout.
• 2. Listen for compressor start: With the system calling for cooling, listen at the outdoor unit. A loud hum with no compressor start suggests a locked rotor or failed start components.
• 3. Inspect contactor: With power off, inspect the contactor for pitted or welded contacts. Replace the contactor if contacts are badly burned or stuck.
• 4. Check capacitor (advanced): Discharge and test the dual run capacitor with a capacitance meter. Replace if outside ±6% of rated µF. A weak capacitor can prevent the compressor from starting and cause repeated trips.

When to call a pro: If lockouts recur after basic checks, the compressor or control board may be failing. These are high-cost components that require professional diagnosis.