Nest Thermostat E73 Error — No Power to Rc Wire Fix

E73 on a Nest thermostat means it can’t detect 24VAC power on the Rc wire — the wire that powers the cooling side of your HVAC system. This is one of the most common Nest errors after a thermostat swap or HVAC service call. The fix is usually a blown fuse at the furnace, which costs about $2 and takes 5 minutes.

What This Error Means

The Nest needs 24VAC power from your HVAC system to operate. Most HVAC systems have two 24V transformers — one for heating (Rh wire) and one for cooling (Rc wire). E73 specifically means the Rc wire isn’t carrying the expected voltage.

Common Causes

• Blown fuse at the furnace/air handler — The most common cause, especially right after replacing a thermostat. When you connect wires to the Nest, a momentary short can blow the 3A fuse on the control board. It’s a safety device designed to fail before the board does.
• Rc wire not connected or loose — The wire came out of the Rc terminal on the Nest base or at the furnace.
• No C-wire, insufficient power — Without a C-wire, the Nest can’t complete the 24VAC circuit and may struggle to power itself reliably.
• Failed 24V transformer — The transformer in the air handler or condenser that produces 24VAC has failed.
• Misconfigured wiring at the HVAC equipment — The Rc wire isn’t connected to the correct terminal at the air handler.

Step-by-Step Fix

Step 1: Check the fuse at the furnace (most common fix)

1. Go to your furnace or air handler — usually in a utility closet, basement, attic, or garage.
2. Turn the power switch on the furnace to Off (there should be a switch on or near the unit — it looks like a light switch).
3. Open the furnace door or access panel. You may need a screwdriver — some panels are held by a clip or two screws.
4. Look for the control board — a circuit board inside the furnace with wires connected to it.
5. Find the fuse on the control board. It’s typically a small glass or ceramic fuse in a holder, labeled “FUSE” or with a symbol. It’s usually 3A 250V.
6. Remove the fuse and test it with a multimeter (continuity setting) — or simply hold it up to a light. A good fuse looks clear or has an intact wire inside. A blown fuse looks cloudy, has a broken wire, or the metal cap has separated.
7. If blown, replace it with the same rating (3A 250V, slow-blow). These are available at any hardware store or online.
8. Replace the fuse, close the access panel, turn the furnace power back on.
9. Check the Nest — E73 should clear if the fuse was the issue.

Step 2: Check the Rc wire connection

At the Nest base (the part that stays on the wall):

1. Remove the Nest thermostat display by pulling it straight toward you.
2. Look at the base — find the Rc connector. Press the button next to it and pull the wire out, then reinsert it until it clicks.

At the furnace: 3. Find where your thermostat wires connect to the control board. Look for the terminal labeled “Rc” (sometimes labeled “R” if the system uses a single R terminal for both heating and cooling). Confirm the wire is seated and the terminal screw is snug.

Step 3: Add a C-wire or Nest Power Connector

If you don’t have a C-wire connected (a “Common” wire that completes the 24V circuit), the Nest may have intermittent power issues that manifest as E73.

The simplest fix is the Nest Power Connector (sometimes called the C-wire adapter) — Google makes it specifically for this situation. It connects to your HVAC equipment and provides power to the Nest without running a new wire.

Step 4: Test the transformer

With the system powered on, use a multimeter set to AC voltage. Measure between the R and C terminals at the furnace control board (or at the thermostat wiring block). You should read 24–28 VAC. Zero volts means the transformer has failed.

Parts You May Need

• Nest Power Connector C-wire adapter
• 3A 250V slow-blow fuse (5-pack)
• 18/5 thermostat wire
• Multimeter for voltage testing

When to Call a Technician

If replacing the fuse and checking wiring connections doesn’t fix E73, the 24V transformer at the furnace or the HVAC equipment itself has a problem. An HVAC technician can test the transformer and trace the wiring quickly.

Related Guides

• Nest Thermostat E1 Error Fix
• Nest Thermostat E74 Error Fix
• Ecobee Thermostat E1 Error Fix