Follow your exact equipment manual. No gas, refrigerant, wiring, internal components or safety-switch bypasses. If access is unsafe or instructions are unclear, stop.
First, rule out a reason to stop
For a gas odor or CO alarm, leave the building and call emergency services from outside. Do not stay inside to finish a checklist or operate switches. Smoke, fire or sparking also requires emergency action. These checks are only for an otherwise safe situation.
Read what the thermostat is asking for
Check the mode: Heat for heating or Cool for cooling. Compare the room temperature with the setpoint; then check whether a schedule, vacation setting or temporary hold explains what you see. Photograph alerts or error codes before changing anything.
Some controls impose a protective delay after a setting change. Follow your owner's manual and allow that delay rather than repeatedly toggling modes or restarting the system. A thermostat showing a call for heat does not prove that the equipment is operating.
Batteries only where the manual permits
If your thermostat uses homeowner-replaceable batteries, follow its instructions for the correct type, orientation and access. Not every thermostat uses batteries. A blank screen may also reflect a power or system-safety issue; it does not automatically mean the thermostat needs replacement.
Stop if access exposes wiring or the procedure is unclear. Do not jump terminals, disconnect wires, change installer settings, factory-reset the control or bypass a safety switch. Do not reset a tripped breaker as part of this checklist; report it to your technician.
Check what is accessible, then hand off
Keep supply registers and return grilles unobstructed. Inspect a user-serviceable filter using the equipment's shutdown and replacement procedure. Do not open electrical or burner compartments to look for a cause.
If normal settings and approved battery or filter care do not resolve the issue, stop experimenting. Note the mode, setpoint, room temperature, displayed code and when the problem began. Mention any water, unusual noise or recent outage. These observations help a technician prepare without guessing which part to replace.
Sources & further reading
General guidance informed by the sources below. Equipment-specific instructions take priority; accessory buying criteria are HVAC PM recommendations, not manufacturer approval.