🌡️Heat Pump

ALERT 34

Lennox · XP21 Dave Lennox Signature Collection 20 SEER Heat Pump

Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump ALERT 34 Error Fix: Auxiliary / Emergency Heat Active

The heat pump has activated supplemental electric heat strips because outdoor conditions reduced heat pump capacity below demand. Informational in normal operation. If auxiliary heat activates at mild temperatures (above 35–40°F), check refrigerant charge, outdoor unit operation, and defrost system. Excessive auxiliary heat use significantly increases electricity costs.

Low severity2 fixes available✨ Free fix available

Quick Answer

Error ALERT 34 on the Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump means: Auxiliary / Emergency Heat Active. The top-ranked community fix is “Check Refrigerant Charge and Defrost System Operation” (hard difficulty, ~90 min, DIY cost: $0-$20). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · heat pump

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 heat pump 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: $200-$350.

Other Lennox Models with Error ALERT 34

XP25 Dave Lennox Signature Collection Variable-Capacity Heat Pump

heat_pump · 2 fixes

Other Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump 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 XP21 Dave Lennox Signature Collection 20 SEER Heat Pump Error ALERT 34

What does error code ALERT 34 mean on a Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump?

Error code ALERT 34 on the Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump means: Auxiliary / Emergency Heat Active. The heat pump has activated supplemental electric heat strips because outdoor conditions reduced heat pump capacity below demand. Informational in normal operation. If auxiliary heat activates at mild temperatures (above 35–40°F), check refrigerant charge, outdoor unit operation, and defrost system. Excessive auxiliary heat use significantly increases electricity costs.

How do I fix error code ALERT 34 on my Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump?

The most successful fix is "Check Refrigerant Charge and Defrost System Operation" (hard difficulty, ~90 minutes). Estimated DIY cost: $0-$20. HomeMD has 2 community-ranked fixes for this error.

Is error code ALERT 34 on a Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump serious?

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

Can I fix Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump error ALERT 34 myself?

Yes — the top-ranked community fix ("Check Refrigerant Charge and Defrost System Operation") is rated hard difficulty and takes about 90 minutes. Tools needed: T20 Torx screwdriver, HVAC manifold gauge set, Multimeter, Vacuum pump.

How much does it cost to repair Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump error ALERT 34?

DIY cost: $0-$20. Professional repair typically costs $200-$350. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump have?

The Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump 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 34 error?

Error code ALERT 34 also appears on 1 other Lennox model: 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 XP21 Dave Lennox Signature Collection 20 SEER Heat Pump have?

The Lennox XP21 Dave Lennox Signature Collection 20 SEER Heat Pump 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 34 error?

Error code ALERT 34 also appears on 1 other Lennox model:

XP25 Dave Lennox Signature Collection Variable-Capacity Heat Pump (heat pump) · 2 fixes

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