Lennox air conditioner outdoor unit runs briefly then shuts down; control board or thermostat shows E201 low-voltage error.

Safety: Turn off power to both indoor and outdoor units before touching wiring. Do not work on live circuits unless you are trained and equipped.

• 1. Inspect low-voltage connections: Remove the blower/furnace panel to access the Lennox control board. Tighten R, C, Y, G, and W terminal screws. Check splices in the low-voltage wiring harness for corrosion or loose wirenuts.
• 2. Check thermostat cable routing: Look for sections of thermostat cable that may be pinched by sheet metal, staples, or doors. Any crushed or cut insulation can cause intermittent shorts between conductors.
• 3. Disconnect accessories: If you have a humidifier, UV light, or zoning system powered from R/C, temporarily disconnect them to see if the error clears. Excessive accessory load can drag down the 24V supply.
• 4. Measure transformer output (advanced): With power on and extreme caution, measure between R and C at the control board while the system is calling for cooling. Voltage should remain near 24VAC. If it drops significantly under load, the transformer may be failing and should be replaced.

Note: Persistent E201 errors often indicate wiring or transformer issues that are best diagnosed with a meter by a professional.