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Lennox · Air conditioner troubleshooting

Outdoor unit will not start - Lennox EL16KC1

What is going on

Thermostat calls for cooling but the compressor and outdoor fan do not run.

Likely causes

  • No 24 VAC Y1 call or common
  • High pressure switch open
  • Low pressure switch open
  • Timed-off control (A4) delay, if installed
  • Refrigerant detection system (RDS) in leak mode
  • Contactor or line voltage problem

Check in this order

  1. Check 24 VAC between Y1 (yellow) and C (black) at the outdoor unit wire connections.
  2. If an A4 timed-off control is used, wait out its delay; Y1 goes to A4 terminal 2 and S4 to terminal 3.
  3. Test high pressure switch S4 it should open at 590 +/- 10 psig and close again at 418 +/- 5 psig.
  4. Check the low pressure switch S87 (opens at 40 +/- 5 psig, resets at 90 +/- 5 psig); a low charge will hold it open.
  5. Check that contactor K1 pulls in and line voltage is within the nameplate range.
  6. If the RDS board LED is flashing blue, the system is in leak mitigation and keeps the compressor off - find the leak or sensor issue first.
  7. Crankcase heater should be powered 24 hours before start-up.

Applies to: Lennox EL16KC1 (Elite) R-454B air conditioners, sizes -018 to -060

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Source: Lennox EL16KC1 Unit Information Corp. 100121 (01-2025), System Operation p.27, Typical Field Wiring pp.30-31, Start-Up p.32, Start-Up Test Procedure p.36. Written in our own words; check the manual for your exact model.

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.