There are a number of reasons why the pilot on your furnace either wont lite or why it won’t stay lit.  Some of these reasons are pretty simple and can be checked by you.  

First let’s look at some of the items you can check:

Thermostat:      

Some thermostats that have weak batteries or are just up there in their years can have a problem with sending a signal to the furnace.  Try changing the batteries and see if that helps.  You should hear the draft inducer motor in the furnace after about 20 to 30 seconds.

Exhaust:              

If your current furnace has an AFUE efficiency rating of 90% or greater it is considered to be high efficiency system.  This unit will be vented through a PVC pipe and can often be found on the side of your house.  Sometimes you will even see two PVC pipes side by side.  The second pipe is used for combustion air.  After you’ve located these pipes make sure they are free of any obstructions like, outside toys propped up on the side of your house, lawn chairs, weeds or ice.  If anything is obstruction these pipes your system will not run.

Air Filter:             

Regularly checking your air filter will not only protect your system it will prevent it from overheating and shutting down.  A dirty filter can restrict airflow to the point of overheating.  Your furnace will be equipped with a high heat limit switch located on the front of your primary heat exchanger.  If this limit switch reaches its maximum heat limit, it will shut your furnace down preventing a catastrophic failure.  

Now let’s take a deeper look at what your technician will need to look at in addition to what you have already checked:

Draft inducer fan:            

After your thermostat sends the signal to ignite your furnace for heat you should hear the draft inducer fan turn on.  It’s this fan’s job to make sure your combustion chamber is free of any excess gas and moisture.  After the fan runs for its proscribed period of time, it will create a small vacuum that will engage the pressure switch causing the furnace to ignite.  If the motor is weak, damaged, has excess moisture or dirt inside, it will not clear the ignition chamber.  If the chamber can’t be cleared it won’t create the vacuum needed causing a failed ignition.

Pressure Switch:              

A pressure switch is a safety limit put in place to prevent ignition.  Its job is to wait until the draft inducer motor has had time to clear the excess or unused gas from the combustion chamber.  Some systems will have more than one.  These will be in place to make sure the furnaces draining system from the heat exchanger is clear.  As listed above, if this switch is faulty or obstructed it will not allow the furnace to ignite or stay lit.

Gas valve:                           

Your gas valve is in place to regulate the amount of gas and to set the limits based on what stage your furnace is in.  Sometimes the chamber in the gas valve can collect junk and prevent the appropriate amount of gas getting to the ignition chamber.

Hot surface ignitor:         

As you can imagine, the one job of the hot surface igniter is to light the gas as its supplied by the gas valve.  Sometimes these can have carbon build up on them or be cracked.  If this is the case your furnace will not light.  

Flame sensor:                   

Much like the hot surface ignitor, the job of the flame sensor is simple.  After the gas is lit by the hot surface ignitor it senses the flame to make sure it will proof.  Once proofed, the flame sensor will send a low voltage signal opening the gas valve into a full stage and the primary blower will turn on to heat your home   

This is a short list of your common problems and also a glimpse into how your furnace works.  There is a long list of other safety circuits and parts that could fail or indicate a more serious problem.  Please do not attempt any of these repairs on your own.  If you think your system is showing any of these symptoms, please call a qualifies technician.