Hi Robert18,
Uh-oh! I'm sorry to hear that your thermostat is showing E73 code. Check on the following:
- If you used AC recently, check your system’s drip pan or drain tubes (skip this step if you only have a heating system)
- With frequent AC use during hot weather, the customer’s drip pan might be full of water, or the drain tube might have a clog. In these cases, the AC or heat pump might shut itself off to prevent an overflow of water. Then, the system might cut the power to the thermostat and cause an E73 power error.
- Check the condition of the HVAC fuse. Occasionally, fuses burn out due to one or more reasons. If your HVAC fuse burns out, the system might cut off the power to the thermostat and cause an E74 power error.
- Turn off the main power to the HVAC system at the breaker box. Note: The HVAC system can have multiple breakers, so make sure you turn them all off.
- Check the furnace and fan and make sure there are no lights that might indicate the system still has power. Note: If there are still lights on the furnace or fan, the system might still have power, and you need to turn off the main power to their home at the breaker.
- Search for the HVAC fuse. It’s usually on the furnace control board. Note: If you need help to find your control board, check your owner’s manual. Fuses can be different for each type of system. You might find a small U-shaped fuse with a number 3 label and is pink or purple colored. You might also find a round fuse.
- Remove and inspect the fuse for damage.
- If the fuse seems burnt or has discoloration, consider a fuse replacement. To learn more about fuse replacement, consult the user guide for the HVAC system or contact a Nest Pro.
- Make sure to reattach any panels on the furnace. Note: The furnace might not work if the customer doesn't reattach the panels.
- Check if you have an energy savings box. Some utility companies install boxes that cut off the power during high-demand (peak) times. These energy savings boxes can cause the error.
- Search for an energy savings box near the electric meter (possibly near their air conditioner).
- Search for a small box with 2 lights on it (one light is red, the other is usually green). This box allows the utility company to cycle the customer’s air conditioner off and on during certain peak times of the day.
- Gather the power readouts from the thermostat.
- Go to Settings > Technical Info > Power and list down the reading for Battery, VOC, VIN and lIN.
- Make sure the thermostat wires are properly stripped and seated in the thermostat base.
- If there’s a common wire connected in the C terminal, remove it.
- Move the power wire to the unused power terminal. If the power wire is in RH, then move it to RC. If it's in RC, then move it to RH.
- If the previous thermostat is available, ask the customer to install it.
It is worth noting that you turn off the power to your HVAC system when you are about to touch some wires on your thermostat base. If you are not comfortable checking on the following, consider contacting a pro.
Let us know how it goes.
Best,
Zoe