Nest Not Turning On Heat: Settings, System Test and Help Codes

First check whether Nest is actually requesting heat. Confirm the current mode and target temperature at the thermostat, not just a saved schedule in the app. Note whether the display shows a heating request while the room remains below the target. A schedule question and a system that ignores an active request need different investigation.

Then check the furnace

Before you change anything on the Nest, go to the furnace. There’s usually a power switch on the side of the unit, looks like a standard light switch. It gets bumped off more often than you’d think. If that’s on, check the circuit breaker for the air handler or furnace. Reset it once if it tripped. Those are the two homeowner checks worth doing here.

Use the test for your Nest generation

Google provides a system-test guide with separate instructions for current Nest Thermostat and fourth-generation Learning Thermostat models, and for Thermostat E and earlier Learning Thermostats. Follow the matching path instead of hunting for a menu your model doesn’t have.

Google also notes that testing may be restricted by outdoor conditions and that some heating systems respond slowly. Forced air responds in minutes. Radiant floors can take 30 minutes or more. Observe the result the thermostat asks you to check, then preserve any message it produces.

Copy the entire help code

Google’s help-code reference distinguishes E73, no power detected at Rc, from E74, no power detected at Rh. Both point at power coming from the equipment, not at the thermostat. Include the complete code, even if two codes look nearly alike.

Leave wiring tests and terminal changes to someone qualified to work on the equipment. Do not bridge wires to make the furnace start.

A dark Nest is a different problem

If the thermostat itself will not respond, use Google’s power and startup troubleshooting. Google identifies a C wire or Power Connector as necessary for some configurations, not every no-heat complaint. Replacing a thermostat before identifying its power issue can leave the original fault in place.

For BAHS diagnostics, send the Nest generation, heat-source type if known, exact message and system-test result. The visit is $99, credited toward an approved repair. We check the equipment response before proposing a thermostat, wiring or heating-system repair.

Key takeaways

• Nest test menus differ between thermostat generations.
• A help code must be read exactly; E73 and E74 identify different power terminals.

Related questions

Need help with your system? Call (925) 999-4095 or book a visit. Repair diagnostics are $99, credited toward the repair. Installation assessments are free.

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA Section 608 Universal certification, and is factory-trained on Daikin (Houston, 2025) and Mitsubishi. He writes from direct field experience, not marketing copy.

Further reading

• Carrier Furnace Status Codes and Greenspeed Heat Pump Fault Codes , A Carrier furnace blinks a two-digit status code on an amber LED. A Greenspeed heat pump or AC sends a code like 18-11 to the Infinity thermostat. Here are both lists from Carrier's service manuals, by model, with the checks you can do before you call.
• Choosing a controller for your Mitsubishi system , Handheld remote, MHK2, wired control or the Comfort app: what to check before adding a controller to a Mitsubishi indoor unit.
• Mitsubishi heat pump lineup: which family fits your home? , Compare FX, GX, JX, HX and WX wall units, SUZ indoor styles and P-Series, then decide between a single-zone system and a multi-zone layout.