Quick answer
Error E73 on a Nest thermostat means the thermostat detects no power on the Rc wire, the 24-volt feed from your cooling system. The Nest is running on its battery and the air conditioner will not start. Check the furnace breaker, the furnace power switch and the condensate drain pan for water. If power is on and E73 stays, book an HVAC technician.
Error E73 on a Nest thermostat means one thing: the thermostat no longer detects 24-volt power on its Rc wire. Rc is the red wire that carries low-voltage power from the cooling side of your system, and without it the Nest can neither call for cooling nor charge itself. Google's help-code table lists E73 as “no power to Rc wire detected” and notes that it tends to show up in hot weather, when the air conditioner is working hardest. Help codes can differ between Nest models, so the wording on your own thermostat screen and in the app is the final word.
Is it safe to leave a Nest showing E73?
E73 is not dangerous by itself, but it is not something to wait out. The thermostat is living on its internal battery; if the power does not come back, the battery drains, the Nest drops off Wi-Fi and then it shuts down. Meanwhile nothing is cooling the house.
The one case that needs attention today is water. If a full condensate pan is what cut the power, the pan is one overflow away from a stained ceiling or a wet basement floor, and the safety switch that tripped is the only thing holding the air conditioner off.
The check to do before you call: power and the drain pan
The 24 volts on Rc comes from a transformer inside the furnace or air handler, not from the outdoor unit. That is why resetting only the air conditioner's breaker rarely clears E73. Work through this list once:
- Find the breaker labelled furnace, air handler or HVAC in the electrical panel. Push it fully off, then back on, and check the outdoor unit's breaker while you are there.
- Check the furnace power switch. It looks like a light switch on or beside the furnace, and storage boxes and laundry baskets bump it off.
- Make sure the furnace's lower door sits flush. A door left ajar releases a safety switch and the transformer loses its power.
- Look at the drain pan and the drain tubing at the indoor coil above the furnace. Standing water, or a small condensate pump with a full reservoir, points to a clogged drain that has tripped a float switch.
- Give the Nest a few minutes after power is restored. If E73 clears, the thermostat recharges without any help from you.
Stop there. Do not bypass a float switch to get the cooling back, and do not swap a blown fuse for a larger one. Both parts exist to protect the equipment and the house.
What causes E73, from most to least likely
- Power is off at the furnace or air handler: a tripped breaker, a switched-off furnace switch or a loose door. It is the first thing to rule out and it costs nothing.
- A clogged condensate drain. In a humid week along Lake Ontario a central air conditioner pulls several litres of water out of the air every day; algae and dust slime block the drain, the pan fills, and a float switch wired into the red wire opens the circuit. Our answer on how to unclog an AC drain line covers the safe part of that job.
- A blown low-voltage fuse on the furnace control board. It is a small 3 A or 5 A blade fuse, and it usually blows because the thin cable running to the outdoor unit has been nicked by a string trimmer, chewed or rubbed through, or because the contactor coil in the condenser has shorted.
- A loose or corroded Rc connection, at the thermostat base or at the control board.
- A failed transformer. This is the least common cause, and it usually follows a short that the fuse did not catch in time.
One wiring detail explains a lot of confusion. A home with a single furnace-and-AC system has one transformer and one red wire, and on a Nest Learning Thermostat that wire can sit in either the Rc or the Rh terminal. If yours sits in Rc, a loss of power shows as E73 in any season; if it sits in Rh you would see error E74 instead.
What a technician does for E73, and what it costs
A technician starts at the control board with a meter, not at the thermostat. The check is whether the transformer is putting out roughly 24 volts AC, whether that voltage survives the fuse and any float switch, and whether it arrives at the Rc terminal on the wall. If the fuse is blown, the real job is finding the short that blew it; a new fuse without that step fails again on the next call for cooling.
- The visit to find the fault: $90–$150, credited when the repair is approved.
- Condensate drain cleaning or trap replacement: $150–$350.
- New condensate pump, installed: $250–$450.
- Most AC electrical fixes, such as a contactor or a shorted low-voltage cable: $150–$450.
- Furnace control board, when the short has damaged it: $500–$1,200.
A transformer replacement is priced on the visit because the part depends on the furnace model. You get the figure in writing before any work starts; our pricing page explains how visits are billed.
If the breaker and switch are on, the pan is dry and E73 is still on the screen, the fault is inside the equipment and it needs a meter. Google's own guidance for a summer E73 is that you will likely need an HVAC professional. ZK Mechanical handles AC repair from Burlington across Oakville, Milton, Hamilton and Mississauga: call (647) 801-1252 or send a request through the contact page. For every other Nest help code, the Nest thermostat error code guide lists them in one place.