Palm Desert winters are mild — until they aren't. When December desert nights drop into the 30s and a furnace won't heat, the four most likely causes rank in this order:

1. Thermostat fan is set to ON, not AUTO

Same rule as summer: FAN ON runs the blower continuously and pulls room-temperature air across a cold heat exchanger between burner cycles. Set the fan to AUTO. The blower will only run when the heat exchanger is warm.

2. The furnace is starting a heating cycle

Modern furnaces take 60–90 seconds from thermostat call to warm air at the register — inducer motor spins up, pressure switch closes, igniter glows, burners light, and only THEN the blower starts pushing warm air. If you feel cold air for the first minute or so, that's normal.

3. Failed ignition or flame sensor

If the burners never light — no whoosh, no glow — the ignition system is bad. Hot-surface igniters (common since the 1990s) crack after about 5–10 years. Flame sensors get coated with oxides and stop detecting flame, which triggers the safety lockout after 3 tries. Both are common, both are affordable. Ballpark $180–$350.

4. Dirty filter → limit switch tripping

A restricted filter starves the heat exchanger of return air, temperatures climb, and the high-limit switch cuts fuel to protect the exchanger. The blower keeps running to cool the metal. Result: cold air blowing after 2–3 minutes of heat. Change the filter and reset the furnace (power off 30 seconds, back on).

What NOT to do

Don't repeatedly reset a locked-out furnace. If it locks out three times in a row, something is genuinely wrong — usually the flame sensor, gas pressure, or a cracked heat exchanger. Repeated resets can flood the burner assembly with gas.
Don't ignore the smell of gas. Ever. Shut the gas valve, open windows, leave, and call SoCalGas at 1-800-427-2200.
Don't tape over the pressure-switch port. Seen it. It's a safety, not a suggestion.

When it's the heat exchanger

A cracked heat exchanger is a rare-but-serious failure that puts combustion byproducts (including carbon monoxide) into your supply air. Symptoms include soot around registers, a persistent CO alarm, unusual burner-flame shape (yellow instead of blue), and rust around the vent. This is a replace-the-furnace call, not a repair — read the decision framework.

If your furnace won't heat, book a diagnostic — we'll pull the panels, meter the ignition system, verify safety switches, and give you a written estimate before ordering parts.

Furnace Blowing Cold Air: Why It Happens (and What Not to Do)