🌡️Thermostat

B03

Nest · Nest Learning Thermostat (3rd Gen)

Nest Nest Learning Thermostat (3rd Gen) B03 Error Fix: Cooling System Will Not Turn On

Thermostat calls for cooling and displays 'Cooling' but no cool air comes from the vents. The thermostat is sending the Y signal correctly but the air conditioner or heat pump is not responding. Check the outdoor unit — is the fan spinning? Is the compressor humming? Check for a tripped breaker at the outdoor disconnect box. Check the capacitor in the outdoor unit (a common failure point). Verify the contactor is pulling in when the thermostat calls for cooling.

critical2 fixes available✨ Free fix available

Quick Answer

Error B03 on the Nest Nest Learning Thermostat (3rd Gen) means: Cooling System Will Not Turn On. The top-ranked community fix is “Test and Replace Outdoor Unit Capacitor” (medium difficulty, ~45 min, DIY cost: $15-$25). 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:

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-$180.

Other Nest Models with Error B03

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

A05

Y1/Y2 Cooling 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

People Also Ask About Nest Nest Learning Thermostat (3rd Gen) Error B03

What does error code B03 mean on a Nest Nest Learning Thermostat (3rd Gen)?

Error code B03 on the Nest Nest Learning Thermostat (3rd Gen) means: Cooling System Will Not Turn On. Thermostat calls for cooling and displays 'Cooling' but no cool air comes from the vents. The thermostat is sending the Y signal correctly but the air conditioner or heat pump is not responding. Check the outdoor unit — is the fan spinning? Is the compressor humming? Check for a tripped breaker at the outdoor disconnect box. Check the capacitor in the outdoor unit (a common failure point). Verify the contactor is pulling in when the thermostat calls for cooling.

How do I fix error code B03 on my Nest Nest Learning Thermostat (3rd Gen)?

The most successful fix is "Test and Replace Outdoor Unit Capacitor" (medium difficulty, ~45 minutes). Estimated DIY cost: $15-$25. HomeMD has 2 community-ranked fixes for this error.

Is error code B03 on a Nest Nest Learning Thermostat (3rd Gen) 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 Nest Nest Learning Thermostat (3rd Gen) error B03 myself?

Yes — the top-ranked community fix ("Test and Replace Outdoor Unit Capacitor") is rated medium difficulty and takes about 45 minutes. Tools needed: T20 Torx screwdriver, Multimeter with capacitance setting, Insulated screwdriver, Needle-nose pliers.

How much does it cost to repair Nest Nest Learning Thermostat (3rd Gen) error B03?

DIY cost: $15-$25. Professional repair typically costs $120-$180. 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), A05 (Y1/Y2 Cooling Wire Not Detected), and 3 more.

Which other Nest models have the B03 error?

Error code B03 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)
A05 — Y1/Y2 Cooling 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)

Which other Nest models have the B03 error?

Error code B03 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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