When a gas furnace in San Leandro stops heating, the cause is rarely the whole furnace. A modern furnace runs through the same short sequence every time it starts, and a no-heat call usually means one step of that sequence failed: the igniter did not glow, the flame sensor could not see the flame, the pressure switch did not prove the venting, or a limit switch shut the burners down because the furnace overheated. A careful diagnosis follows the sequence until it finds the step that fails.
How a furnace starts, and where it fails
- The thermostat calls for heat. No call, no heat: wiring, batteries or settings.
- The inducer fan starts to push combustion gases out through the vent.
- The pressure switch confirms the venting is clear. A blocked vent, a cracked hose or a failing inducer stops the start here.
- The igniter heats up (or a spark igniter fires). Hot surface igniters wear out and crack.
- The gas valve opens and the burners light.
- The flame sensor proves the flame. If it cannot, the gas valve closes within seconds and the furnace tries again, then locks out for a while.
- The blower starts and sends warm air through the ducts. A weak blower or a clogged filter lets the furnace overheat, and the limit switch shuts it down.
Most furnaces show a blinking diagnostic light through a small window in the lower door. Counting the blinks and reading the code chart on the door narrows the search before the panel even comes off.
Common symptoms and what they usually mean
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Lights, then shuts off after a few seconds | Dirty flame sensor | Cleaning or replacing the sensor |
| Inducer runs, but no ignition | Pressure switch, blocked vent or failed igniter | Testing each, then the failed part |
| Blower runs but the air is cold | Ignition lockout, gas supply or limit trip | Finding why the burners are off |
| Heats for a few minutes, then cycles off | Clogged filter, blocked returns or a weak blower | Airflow correction or blower repair |
| Loud boom when the burners light | Delayed ignition from dirty burners | Burner cleaning and adjustment |
| Squealing or grinding | Worn blower motor bearings or a damaged wheel | Motor or wheel replacement |
| Pilot light keeps going out (older furnaces) | Dirty pilot or failing thermocouple | Cleaning or thermocouple replacement |
| Thermostat calls but nothing happens | Thermostat, wiring, transformer or fuse | Electrical testing |
If the house is simply not getting warm, our page on a furnace that is not heating walks through the common cases in more detail.
Gas, electric and oil furnaces
Most homes here heat with a gas furnace, in a hall closet, a garage, an attic or under the floor. We also repair electric furnaces, where the usual faults are failed heating elements, sequencers and limits, and oil-fired furnaces. Every repair visit includes a check of the safety controls and the venting before the furnace is put back into service.
Safety first
- If you smell gas, do not switch lights or appliances on or off. Leave the house and call your gas utility’s emergency line or 911 from outside.
- If a carbon monoxide alarm sounds, get everyone outside and call 911.
- Do not keep resetting a furnace that trips its rollout switch or locks out repeatedly. Those switches stop the furnace for a reason, often a venting or burner problem that needs to be found.
Before you call: safe checks
- The thermostat is set to Heat and above room temperature, with fresh batteries if it uses them.
- The filter is not clogged; a blocked filter alone can make a furnace overheat and shut down.
- The service switch near the furnace, which looks like a light switch, is on, and so is the breaker.
- The blower door is fully closed; a safety switch keeps the furnace off when it is loose.
- Write down the blink code before turning the power off, since a power cycle clears it.
What affects the price
Most furnace repairs fall between $89 and $500. Flame sensors, igniters, thermocouples and pressure switch hoses sit at the lower end and are usually fixed on the first visit. Inducer motors, blower motors, gas valves and control boards cost more, and access to an attic or crawlspace furnace adds time. The diagnostic fee is waived when you go ahead with the repair, you see the price before work starts, and repairs carry a 90-day warranty. The furnace repair cost guide breaks the numbers down further.
Repair or replace?
Repair makes sense when one part has failed and the furnace is otherwise sound. A cracked heat exchanger, repeated major repairs on an older unit, or a furnace that cannot keep the house warm are when replacement deserves a serious look. Our repair-or-replace guide walks through the decision, and if replacement wins, see furnace installation.
Local notes
Heating season in San Leandro, San Lorenzo and Castro Valley runs mostly from November to March, and the first cold snap brings a wave of calls from furnaces that sat idle since spring. A brief dusty smell on the first run of the year is normal; a smell that does not fade, or any smell of gas, is not. A fall furnace tune-up catches dirty flame sensors and weak igniters before the cold arrives.