🌡️Thermostat

A05

Nest · Nest Learning Thermostat (3rd Gen)

Nest Nest Learning Thermostat (3rd Gen) A05 Error Fix: Y1/Y2 Cooling Wire Not Detected

Thermostat cannot detect the cooling wire (Y1 or Y2). The air conditioning system will not activate. The wire may be disconnected at the thermostat base plate or at the outdoor unit control board. Check the Y1 terminal on both ends. On heat pump systems, the Y wire may be labeled differently — consult the wiring diagram for the specific HVAC system.

High severity2 fixes available✨ Free fix available

Quick Answer

Error A05 on the Nest Nest Learning Thermostat (3rd Gen) means: Y1/Y2 Cooling Wire Not Detected. The top-ranked community fix is “Test Cooling Wire Continuity and Replace Damaged Wire” (medium difficulty, ~40 min, DIY cost: $10-$20). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · thermostat

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 thermostat repairs can be done safely at home, consider hiring a qualified technician if:

This is a high severity error — continued use may cause damage or a safety hazard
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: $120-$200.

Other Nest Models with Error A05

Nest Learning Thermostat (4th Gen)

thermostat · 2 fixes

Nest Thermostat (2020)

thermostat · 2 fixes

Nest Thermostat E

thermostat · 2 fixes

Other Nest Nest Learning Thermostat (3rd Gen) Error Codes

A01

No Power / Low Battery

2 fixes

A02

'Delayed' Message on Display

2 fixes

A03

No Power to Rh/Rc Wire

2 fixes

A04

W1/W2 Heating Wire Not Detected

2 fixes

A06

G (Fan) Wire Not Detected

2 fixes

A07

O/B (Heat Pump Reversing Valve) Wire Not Detected

2 fixes

A08

Wiring Error / Incompatible Wiring

2 fixes

A09

System Not Responding / No Equipment Detected

2 fixes

People Also Ask About Nest Nest Learning Thermostat (3rd Gen) Error A05

What does error code A05 mean on a Nest Nest Learning Thermostat (3rd Gen)?

Error code A05 on the Nest Nest Learning Thermostat (3rd Gen) means: Y1/Y2 Cooling Wire Not Detected. Thermostat cannot detect the cooling wire (Y1 or Y2). The air conditioning system will not activate. The wire may be disconnected at the thermostat base plate or at the outdoor unit control board. Check the Y1 terminal on both ends. On heat pump systems, the Y wire may be labeled differently — consult the wiring diagram for the specific HVAC system.

How do I fix error code A05 on my Nest Nest Learning Thermostat (3rd Gen)?

The most successful fix is "Test Cooling Wire Continuity and Replace Damaged Wire" (medium difficulty, ~40 minutes). Estimated DIY cost: $10-$20. HomeMD has 2 community-ranked fixes for this error.

Is error code A05 on a Nest Nest Learning Thermostat (3rd Gen) serious?

This error is rated high severity. You should address it promptly — continued use could cause damage or safety concerns.

Can I fix Nest Nest Learning Thermostat (3rd Gen) error A05 myself?

Yes — the top-ranked community fix ("Test Cooling Wire Continuity and Replace Damaged Wire") is rated medium difficulty and takes about 40 minutes. Tools needed: Multimeter, Wire stripper, Flathead screwdriver, Needle-nose pliers.

How much does it cost to repair Nest Nest Learning Thermostat (3rd Gen) error A05?

DIY cost: $10-$20. Professional repair typically costs $120-$200. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Nest Nest Learning Thermostat (3rd Gen) have?

The Nest Nest Learning Thermostat (3rd Gen) has 8 other known error codes, including: A01 (No Power / Low Battery), A02 ('Delayed' Message on Display), A03 (No Power to Rh/Rc Wire), A04 (W1/W2 Heating Wire Not Detected), A06 (G (Fan) Wire Not Detected), and 3 more.

Which other Nest models have the A05 error?

Error code A05 also appears on 3 other Nest models: Nest Learning Thermostat (4th Gen), Nest Thermostat (2020), Nest Thermostat E. The fix may vary by model, so check each model's specific page for tailored repair steps.

What other error codes does the Nest Nest Learning Thermostat (3rd Gen) have?

The Nest Nest Learning Thermostat (3rd Gen) has 8 other known error codes:

A01 — No Power / Low Battery (2 fixes)
A02 — 'Delayed' Message on Display (2 fixes)
A03 — No Power to Rh/Rc Wire (2 fixes)
A04 — W1/W2 Heating Wire Not Detected (2 fixes)
A06 — G (Fan) Wire Not Detected (2 fixes)
A07 — O/B (Heat Pump Reversing Valve) Wire Not Detected (2 fixes)
A08 — Wiring Error / Incompatible Wiring (2 fixes)
A09 — System Not Responding / No Equipment Detected (2 fixes)

Which other Nest models have the A05 error?

Error code A05 also appears on 3 other Nest models:

Nest Learning Thermostat (4th Gen) (thermostat) · 2 fixes
Nest Thermostat (2020) (thermostat) · 2 fixes
Nest Thermostat E (thermostat) · 2 fixes

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