🌡️Thermostat

B04

Nest · Nest Thermostat (2020)

Nest Nest Thermostat (2020) B04 Error Fix: Heat Pump Heating/Cooling Reversed

Heat pump produces cold air when the thermostat calls for heat, or warm air when calling for cool. The O/B reversing valve wire setting is configured incorrectly in the thermostat's equipment settings. Go to Settings > Equipment > Heat Pump and toggle the O/B orientation. Most systems use 'O' (energize on cool), but some Rheem and Ruud systems use 'B' (energize on heat). The physical HVAC system is working correctly — only the reversing valve direction needs to be changed in software.

High severity2 fixes available✨ Free fix available

Quick Answer

Error B04 on the Nest Nest Thermostat (2020) means: Heat Pump Heating/Cooling Reversed. The top-ranked community fix is “Verify and Correct Reversing Valve Wiring and Thermostat Configuration” (medium difficulty, ~40 min, DIY cost: $0-$10). 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: $100-$160.

Other Nest Models with Error B04

Nest Learning Thermostat (3rd Gen)

thermostat · 2 fixes

Nest Learning Thermostat (4th Gen)

thermostat · 2 fixes

Nest Thermostat E

thermostat · 2 fixes

Other Nest Nest Thermostat (2020) 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 Thermostat (2020) Error B04

What does error code B04 mean on a Nest Nest Thermostat (2020)?

Error code B04 on the Nest Nest Thermostat (2020) means: Heat Pump Heating/Cooling Reversed. Heat pump produces cold air when the thermostat calls for heat, or warm air when calling for cool. The O/B reversing valve wire setting is configured incorrectly in the thermostat's equipment settings. Go to Settings > Equipment > Heat Pump and toggle the O/B orientation. Most systems use 'O' (energize on cool), but some Rheem and Ruud systems use 'B' (energize on heat). The physical HVAC system is working correctly — only the reversing valve direction needs to be changed in software.

How do I fix error code B04 on my Nest Nest Thermostat (2020)?

The most successful fix is "Verify and Correct Reversing Valve Wiring and Thermostat Configuration" (medium difficulty, ~40 minutes). Estimated DIY cost: $0-$10. HomeMD has 2 community-ranked fixes for this error.

Is error code B04 on a Nest Nest Thermostat (2020) 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 Thermostat (2020) error B04 myself?

Yes — the top-ranked community fix ("Verify and Correct Reversing Valve Wiring and Thermostat Configuration") is rated medium difficulty and takes about 40 minutes. Tools needed: Phillips screwdriver, Multimeter.

How much does it cost to repair Nest Nest Thermostat (2020) error B04?

DIY cost: $0-$10. Professional repair typically costs $100-$160. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Nest Nest Thermostat (2020) have?

The Nest Nest Thermostat (2020) 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 B04 error?

Error code B04 also appears on 3 other Nest models: Nest Learning Thermostat (3rd Gen), Nest Learning Thermostat (4th Gen), 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 Thermostat (2020) have?

The Nest Nest Thermostat (2020) 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 B04 error?

Error code B04 also appears on 3 other Nest models:

Nest Learning Thermostat (3rd Gen) (thermostat) · 2 fixes
Nest Learning Thermostat (4th Gen) (thermostat) · 2 fixes
Nest Thermostat E (thermostat) · 2 fixes

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