❄️Air Conditioner

ALERT 21

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 21 Error Fix: Compressor Lockout

The compressor has been locked out due to repeated fault conditions (typically 3 consecutive trips). The unit will not attempt restart until lockout is manually cleared. To reset: cycle power at the outdoor disconnect (wait 30 seconds). Investigate and resolve underlying cause before resetting.

critical2 fixes available✨ Free fix available

Quick Answer

Error ALERT 21 on the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner means: Compressor Lockout. The top-ranked community fix is “Diagnose and Repair Compressor Lockout Causes Including Electrical and Refrigerant Issues” (hard difficulty, ~90 min, DIY cost: $0-$20). 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: $300-$600.

Other Lennox Models with Error ALERT 21

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 21

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

Error code ALERT 21 on the Lennox XC21 Dave Lennox Signature Collection 21 SEER Variable-Capacity Air Conditioner means: Compressor Lockout. The compressor has been locked out due to repeated fault conditions (typically 3 consecutive trips). The unit will not attempt restart until lockout is manually cleared. To reset: cycle power at the outdoor disconnect (wait 30 seconds). Investigate and resolve underlying cause before resetting.

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

The most successful fix is "Diagnose and Repair Compressor Lockout Causes Including Electrical and Refrigerant Issues" (hard difficulty, ~90 minutes). Estimated DIY cost: $0-$20. HomeMD has 2 community-ranked fixes for this error.

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

This error is rated critical severity. It is generally not urgent and can often be resolved with a simple DIY fix.

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

Yes — the top-ranked community fix ("Diagnose and Repair Compressor Lockout Causes Including Electrical and Refrigerant Issues") is rated hard difficulty and takes about 90 minutes. Tools needed: Multimeter, Manifold gauge set, Electronic leak detector, T20 Torx screwdriver, Nut driver set.

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

DIY cost: $0-$20. Professional repair typically costs $300-$600. 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 21 error?

Error code ALERT 21 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 21 error?

Error code ALERT 21 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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