Furnace Clicking but Not Turning On? What Each Click Means

Reviewed by PRO MAX HVAC & Appliance Repair technicians, EPA 608 certified

A furnace clicking but not turning on is running its startup sequence and stopping at a step it cannot prove. Each click marks one step. Check the thermostat, the filter, the door panel and the condensate drain hose first, then read the status light, because those five checks find the problem on most first-cold-morning calls.

The filter is the easiest place to start. ENERGY STAR recommends checking your furnace filter every month and changing it at least every three months, and in the Sacramento area a filter often sits through a whole summer of AC use before the furnace runs again. A clogged filter starves the furnace of air, and modern controls shut it down rather than let it run that way.

What Does a Normal Startup Sound Like?

A normal start makes two to four distinct clicks in the first minute, then the burners light and the blower follows. Knowing the order tells you where yours stops.

First, the thermostat calls for heat and a relay on the control board clicks. Second, the small inducer fan starts, a steady whir that clears the vent. Third, the pressure switch clicks closed to prove the vent is clear. Fourth, the igniter either glows orange for 15 to 45 seconds or sparks with a rapid ticking. Fifth, the burner valve clicks open and the burners light. About a minute later the main blower starts and warm air reaches the vents.

If you hear the relay click and nothing else, the problem is early. If the inducer runs but you never hear the second click, the pressure switch is not closing. If you hear ticking or see the glow but the burners never light, the furnace is failing at ignition. An early stop is often something you can fix yourself; a failure at ignition almost always needs a technician.

Check the Thermostat and the Door Panel

Start at the thermostat, because a dead or confused thermostat produces a click with no follow-through. Set it to heat, not auto, and raise the setpoint a few degrees above room temperature. Replace the batteries if the screen is dim or blank.

Then look at the furnace itself. The lower front panel presses a small switch when it is seated. If the panel was bumped while someone changed the filter, the furnace hears the thermostat, clicks its relay and refuses to go further. Press the panel firmly back into place.

Replace the Filter

Turn the furnace switch off, then pull the filter and hold it up to a light. If you cannot see through it, replace it with the same size. A badly clogged filter sends the furnace into a short run, an overheat shutdown and a restart, which sounds like a furnace clicking on and off every few minutes. The fix costs the price of a filter.

Check the Condensate Drain on a High-Efficiency Furnace

If your furnace vents through white plastic pipe, it is a high-efficiency model that makes water, and that water drains through a small trap and hose. The trap always holds some water; that is normal and it has to stay that way. When the hose kinks or sludge blocks the trap, water backs up into the pressure switch tubing and the furnace refuses to light. The inducer runs, the second click never comes, and the cycle repeats.

Here is a typical first-cold-morning call in Citrus Heights. The house has a 2005 high-efficiency furnace in the garage, and the AC coil on top of it drains through the same line. The furnace hums and clicks but never lights. The clear trap is clogged with the slime that grew there all summer from AC condensate. A technician cleans the trap, flushes the line and refills the trap with water, and the furnace lights on the next try.

Your part is simple: with the furnace switch off, straighten a kinked or sagging drain hose and make sure its end is not blocked. Leave the trap itself to a technician, because it has to be cleaned and refilled the way your model's manual specifies. Our view: if your high-efficiency furnace and AC share a drain, have the trap cleaned every fall before you need heat.

Read the Status Light, Then Reset Once

Most furnaces made in the last 20 years have a control board light visible through a small window in the lower panel. It blinks a pattern. Count the blinks, then match them to the chart printed on the inside of the furnace door. That code tells you, and the technician, exactly which step failed.

After a few failed tries, most furnaces lock out for an hour or more. You can reset that once by turning the furnace switch off for a minute and back on. If it clicks through the same failure again, stop resetting. Repeated restarts do not fix a failed part. The U.S. Department of Energy recommends regular professional maintenance for heating equipment, and a furnace that keeps locking out is overdue for it.

When to Call a Pro

Call a technician when the thermostat, filter, door panel and condensate drain hose all check out and the furnace still clicks without lighting. The usual parts at that point are a pressure switch or its tubing, a worn hot surface igniter, a dirty flame sensor that lets the burners light and then shuts them off, an inducer motor that runs slow, or a control board relay that has started to fail. Each one needs testing with a meter, and the blink code you counted saves time on the visit.

PRO MAX HVAC & Appliance Repair handles furnace repair across the Sacramento area, including Carmichael and Citrus Heights. Call (916) 234-5925 or book through our online booking page, and tell us the blink code if you have one.

Check a furnace that clicks but will not turn on

1. 1

   Confirm the thermostat call

   Set the thermostat to heat, a few degrees above room temperature, and replace its batteries if the display is dim.

2. 2

   Replace a dirty filter

   Turn the furnace switch off and slide out the filter; if you cannot see light through it, put in a new one of the same size.

3. 3

   Seat the door panel

   Make sure the lower front panel is fully pressed in, because a safety switch behind it keeps the furnace off when it is loose.

4. 4

   Read the status light

   Count the blinks on the control board light through the sight window and match the pattern to the chart on the inside of the door.

5. 5

   Check the condensate drain hose

   On a high-efficiency furnace with white plastic vent pipes, turn the furnace switch off and straighten any kinked or sagging drain hose; leave cleaning the trap itself to a technician.

Sources

• ENERGY STAR — Heating and cooling
• U.S. Department of Energy