Nest Nest Learning Thermostat (4th Gen) A03 Error Fix: No Power to Rh/Rc Wire

Error code A03 on the Nest Nest Learning Thermostat (4th Gen) means: No Power to Rh/Rc Wire. Thermostat displays 'No power to Rh wire' or 'No power to Rc wire.' The thermostat is not receiving 24VAC power from the HVAC system transformer through the R wire. This can be caused by a tripped breaker, blown fuse on the furnace control board, turned-off HVAC system switch, disconnected wire at the furnace terminal, or a failed HVAC transformer. Check the furnace door switch (many furnaces cut power when the door is removed).

The most successful fix is "Test and Replace HVAC Transformer or Repair Wiring" (hard difficulty, ~60 minutes). Estimated DIY cost: $50-$100. HomeMD has 2 community-ranked fixes for this error.

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

Yes — the top-ranked community fix ("Test and Replace HVAC Transformer or Repair Wiring") is rated hard difficulty and takes about 60 minutes. Tools needed: Phillips screwdriver, Multimeter, Wire stripper, Needle-nose pliers.

DIY cost: $50-$100. Professional repair typically costs $150-$250. Many homeowners save money by trying the top-ranked community fix first.

The Nest Nest Learning Thermostat (4th Gen) has 8 other known error codes, including: A01 (No Power / Low Battery), A02 ('Delayed' Message on Display), A04 (W1/W2 Heating Wire Not Detected), A05 (Y1/Y2 Cooling Wire Not Detected), A06 (G (Fan) Wire Not Detected), and 3 more.

Error code A03 also appears on 3 other Nest models: Nest Learning Thermostat (3rd Gen), Nest Thermostat (2020), Nest Thermostat E. The fix may vary by model, so check each model's specific page for tailored repair steps.