Nest E73: no power to the Rc wire, and the cooling side behind it

What Does Error E73 Mean?

These codes report that the thermostat is no longer receiving 24 volt power on a named wire. Google groups E3, E23, and E73 as no power to the Rc wire, E4, E24, and E74 as no power to the Rh wire, and E195 as no power to the R wire, so the letter in the message identifies which transformer feed went quiet rather than which component failed. Applies to: Google Nest Learning Thermostat and Nest Thermostat This page is written around the Rc group of Nest help codes. HVAC Bench covers the Rh group, headed by E74, on its own page, and the two should not be read across: on a two-transformer installation they point at different equipment in different parts of the building. Whether your system has separate Rc and Rh feeds at all is a property of the installed equipment. Owner-safe checks: Check the breakers for the heating and the cooling equipment and the service switch beside the furnace or air handler. | Replace the air filter if it is dirty, which Google includes in its own troubleshooting sequence. | Look for water in the drip pan or a blocked drain tube under the indoor unit. | Turn off power to both the heating and cooling equipment before touching any thermostat wire. | With power off, confirm each thermostat wire is straight, clean, and inserted until the connector button stays pressed down. | With power off, check whether Rc and Rh were crossed if the thermostat was recently installed, and correct them if so. Qualified technician: A blown or blackened system fuse, whose cause needs to be found before replacement. | A condensate blockage that needs the drain line cleared. | An absent 24 volt supply once breakers, switches, and wiring have been confirmed correct. | Any equipment panel that would need to be opened to reach a transformer or a safety device. | Repeated recurrence of the same code after the drainage or filter has been corrected.