HVACopedia

Lennox · Heat pump

71504 Suction temperature sensor incorrect reading

Low severity

What the control is telling you

Measured suction temperature (ST) outside expected range; no major system impact. (S40 system diagnostics) S30/S40 thermostat alert code 71504.

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

Likely causes

  • See description

Test and repair steps

  1. Check the sensor and replace if failed.
  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.49. 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.