HVACopedia

Lennox · Air conditioner

71518 Compressor failed to start

High severity

What the control is telling you

Liquid to suction pressure ratio outside the expected range. (S40 system diagnostics) S30/S40 thermostat alert code 71518.

Applies to: Lennox communicating air conditioners incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Likely causes

  • See description

Test and repair steps

  1. Service the compressor (power, capacitor/inverter, windings).
  2. Clears automatically once fixed.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.50. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.