❄️Air Conditioner

ALERT 71

Lennox · XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner

Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner ALERT 71 Error Fix: Outdoor Ambient Temperature Sensor Fault

The outdoor ambient temperature sensor (thermistor) is reading out of range. This sensor helps optimize compressor staging and defrost timing. NTC thermistor should read approximately 10K–15K ohms at 77°F. Check sensor connector and wiring. Replace if reading is open or shorted.

Medium severity2 fixes available✨ Free fix available

Quick Answer

Error ALERT 71 on the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner means: Outdoor Ambient Temperature Sensor Fault. The top-ranked community fix is “Test and Replace Outdoor Ambient Temperature Sensor Thermistor” (medium difficulty, ~45 min, DIY cost: $10-$15). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · air conditioner

Content is based on manufacturer service documentation, community repair experience, and OEM technical bulletins. Always verify with your owner's manual.Learn about our editorial standards.

When to Call a Professional

While many air conditioner repairs can be done safely at home, consider hiring a qualified technician if:

The repair involves electrical wiring, gas lines, or refrigerant
You've tried the top-ranked fix and the error persists
You smell gas, see sparking, or notice water leaking
The repair requires specialized tools you don't have
Your appliance is still under manufacturer warranty

Average professional repair cost for this issue: $100-$180.

Other Lennox Models with Error ALERT 71

XC25 Dave Lennox Signature Collection 26 SEER Variable-Capacity Air Conditioner

air_conditioner · 2 fixes

XP21 Dave Lennox Signature Collection 20 SEER Heat Pump

heat_pump · 2 fixes

XP25 Dave Lennox Signature Collection Variable-Capacity Heat Pump

heat_pump · 2 fixes

Other Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner Error Codes

1 FLASH YELLOW

High Pressure Switch Open

2 fixes

2 FLASHES YELLOW

Low Pressure Switch Open

2 fixes

3 FLASHES YELLOW

Compressor Thermal Overload

2 fixes

4 FLASHES YELLOW

Short-Cycle Protection

2 fixes

ALERT 1

Communication Loss — Outdoor Unit

2 fixes

ALERT 11

High Pressure Protection — Outdoor Unit

2 fixes

ALERT 12

Low Pressure Protection — Outdoor Unit

2 fixes

ALERT 14

Compressor Discharge Temperature High

2 fixes

People Also Ask About Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner Error ALERT 71

What does error code ALERT 71 mean on a Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner?

Error code ALERT 71 on the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner means: Outdoor Ambient Temperature Sensor Fault. The outdoor ambient temperature sensor (thermistor) is reading out of range. This sensor helps optimize compressor staging and defrost timing. NTC thermistor should read approximately 10K–15K ohms at 77°F. Check sensor connector and wiring. Replace if reading is open or shorted.

How do I fix error code ALERT 71 on my Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner?

The most successful fix is "Test and Replace Outdoor Ambient Temperature Sensor Thermistor" (medium difficulty, ~45 minutes). Estimated DIY cost: $10-$15. HomeMD has 2 community-ranked fixes for this error.

Is error code ALERT 71 on a Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner serious?

This error is rated medium severity. It should be addressed soon, but your appliance may still function in a limited capacity.

Can I fix Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner error ALERT 71 myself?

Yes — the top-ranked community fix ("Test and Replace Outdoor Ambient Temperature Sensor Thermistor") is rated medium difficulty and takes about 45 minutes. Tools needed: Digital multimeter, Needle nose pliers, Phillips screwdriver, Voltage tester.

How much does it cost to repair Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner error ALERT 71?

DIY cost: $10-$15. Professional repair typically costs $100-$180. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner have?

The Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner has 8 other known error codes, including: 1 FLASH YELLOW (High Pressure Switch Open), 2 FLASHES YELLOW (Low Pressure Switch Open), 3 FLASHES YELLOW (Compressor Thermal Overload), 4 FLASHES YELLOW (Short-Cycle Protection), ALERT 1 (Communication Loss — Outdoor Unit), and 3 more.

Which other Lennox models have the ALERT 71 error?

Error code ALERT 71 also appears on 3 other Lennox models: XC25 Dave Lennox Signature Collection 26 SEER Variable-Capacity Air Conditioner, XP21 Dave Lennox Signature Collection 20 SEER Heat Pump, XP25 Dave Lennox Signature Collection Variable-Capacity Heat Pump. The fix may vary by model, so check each model's specific page for tailored repair steps.

What other error codes does the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner have?

The Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner has 8 other known error codes:

1 FLASH YELLOW — High Pressure Switch Open (2 fixes)
2 FLASHES YELLOW — Low Pressure Switch Open (2 fixes)
3 FLASHES YELLOW — Compressor Thermal Overload (2 fixes)
4 FLASHES YELLOW — Short-Cycle Protection (2 fixes)
ALERT 1 — Communication Loss — Outdoor Unit (2 fixes)
ALERT 11 — High Pressure Protection — Outdoor Unit (2 fixes)
ALERT 12 — Low Pressure Protection — Outdoor Unit (2 fixes)
ALERT 14 — Compressor Discharge Temperature High (2 fixes)

Which other Lennox models have the ALERT 71 error?

Error code ALERT 71 also appears on 3 other Lennox models:

XC25 Dave Lennox Signature Collection 26 SEER Variable-Capacity Air Conditioner (air conditioner) · 2 fixes
XP21 Dave Lennox Signature Collection 20 SEER Heat Pump (heat pump) · 2 fixes
XP25 Dave Lennox Signature Collection Variable-Capacity Heat Pump (heat pump) · 2 fixes

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