No Heat on a Cold Modesto Night: The First Ten Minutes

Updated 2026-10 · Modesto, CA

Minute one: is anything unsafe?

Before anything else, two questions.

Do you smell gas?

The rotten-egg smell added to natural gas. If yes, do not touch light switches, the thermostat or the furnace. Get everyone out, leave the door open, and call PG&E or 911 from outside or from a neighbour's house. Do not go back in until the utility says so. Nothing else on this page applies until then.

Is a carbon monoxide alarm sounding?

Same answer: everyone out, call 911 from outside. Carbon monoxide has no smell, and headaches, nausea or dizziness in more than one person in the house are a warning even without an alarm. California has required CO alarms in homes with gas appliances since 2011; if yours is missing or past its date, replace it this week.

Minutes two to five: the thermostat and the power

• Thermostat set to heat, set a few degrees above the room temperature, and the screen alive. Replace the batteries if it is blank or dim.
• Thermostat fan set to auto, not on. On runs the blower all the time and makes a working furnace feel like it is blowing cold air.
• The furnace power switch, which looks like a light switch on or near the unit, set to on. These get flipped by accident in garages.
• The furnace breaker in the panel. Reset it once. If it trips again, leave it off.
• The furnace door or front panel fully seated. A safety switch behind it keeps everything off when the panel is loose.

Minutes five to eight: the filter and the vents

Pull the filter. If it is grey and bowed, the furnace may have shut itself down on its high-limit switch because it could not move enough air. Put in a clean one, or run briefly without one only if you have no spare (and replace it tomorrow). Then check that supply registers and the return grille are not blocked by furniture or closed. On a high-efficiency furnace, check outside that the two white plastic pipes are not blocked by leaves or a bird's nest.

Minutes eight to ten: watch it try

Turn the thermostat up and watch and listen, from the furnace if you can see it. The normal order is: the inducer motor hums, a click, an igniter glows orange or a spark ticks, the burners light, and a minute later the blower starts. Where it stops is the most useful thing you can tell the person you call.

Many furnaces have a small window with a blinking light. Count the blinks and look for the code chart on the inside of the panel door. Write it down.

Then stop

If those checks do not bring it back, stop there. Resetting the furnace again and again, taking panels off or jumping switches is how a small repair becomes a big one, and some faults are safety lockouts doing their job. Write down the furnace's age (often on the label inside the door), where it is, what it did when it tried to start, and any code. Then call.

Keeping warm meanwhile

A Modesto house in December loses heat slowly enough that a night without the furnace is uncomfortable rather than dangerous for most people. Close doors to unused rooms, use extra blankets, and if you use a space heater, plug it straight into the wall and keep three feet clear around it. Never use a gas oven, a barbecue or a camp heater indoors for warmth. If someone in the house is very young, very old or unwell, tell the company when you call.

Where to go next

• Furnace repair in Modesto
• Heat pump repair, including defrost in the fog
• What the repair is likely to cost