Please read first for furnace repairs.

The furnace is not an install and forget appliance. It is intended to be checked and cleaned at least every other year. Allowing it to run until there is a problem is not a recommended practice. Most natural gas suppliers have excellent support technicians. Don't hesitate to contact them.

This sticky is primarily about gas furnaces. Oil furnaces are somewhat different and are handled on a case by case basis in the individuals thread.

Most newer furnaces use a control board that has a fault LED on it. Some use two fault LED's. Many furnaces have a sight glass in the blower door where you can look in to see the flashing LED. Other furnaces you need to remove the blower door and hold the safety switch in to get the furnace to operate. Either way.... removing the door cuts power to the furnace and the codes are erased. Only a very few high end furnaces will store trouble codes. There is usually a list of the codes on the back of the blower door. Get the code list before opening the door.

Always check a furnace for a fault LED on the control board. This is the place to start.

Older furnaces may not have a control board with a fault LED. For this type of control board you'll need to recognize where the heating cycle is failing.

Furnaces follow a very logical procedure. Letting us know where the failure occurs speeds help.

For a hot surface igniter type furnace.
1) Thermostat calls for heat.
2) Draft inducer starts running.
3) Pressure switch proves vacuum from draft inducer.
4) Igniter starts heating as long as pressure switch has proven.
5) Gas valve opens after igniter heats for 15-45 seconds.
6) Burner will remain lit if flame sensor senses flame within 5-7 seconds.
7) If flame is not sensed reliably.... the gas valve will close and the cycle starts again.
8) Typically after 3-4 attempts the control board will go into lockout for at least one hour.
.....Some will reset in one hour others must be reset by turning power off and then back on.

The gas valve should never be reopening randomly.

If the burner only lights for a few seconds.... the first thing to check would be the flame sensor. The flame sensor is nothing more than a stainless steel rod that must be cleaned. My choice of cleaning is to use a green Scotchbrite pad. The rod needs to be polished and the glass oxide removed. Steel wool is also ok. Sandpaper puts scratches in the rod which cause the oxides to accumulate faster and is not recommended. The rod must be engulfed completely in the flame. If you still have problems that point to a flame sense issue.... you need to get a meter that has a ua (microamp) scale and put it in series with the flame rod. It should show better than 1ua. Typically 1.5-3ua

For an intermittent spark pilot ignition.
1) Thermostat calls for heat.
2) Draft inducer starts running.
3) Pressure switch proves vacuum from draft inducer.
4) Pilot valve opens and spark igniter starts as long as pressure switch has proven.
5) Pilot lights successively, spark ignition shuts off gas valve opens.
6) Burner will remain lit if flame sensor senses flame within 5-7 seconds.
7) If flame is not sensed reliably.... the gas valve will close and the cycle starts again.
8) Typically after 3-4 attempts the control board will go into lockout for at least one hour.
.....Some will reset in one hour others must be reset by turning power off and then back on.

This type of system typically use a combined spark igniter/flame sensor. If this is dirty the burner will shut off and the pilot will try to relight. This could repeat over and over many times before a default occurs. Often the best repair is to replace the pilot assembly. A spark ignition problem is usually best handled in the individuals thread.

If you get a "pressure switch stuck open" code it just means that the pressure switch is not responding to the draft inducer. It's not necessarily stuck open. It's just not closing. Pressure switches are extremely reliable and are hardly ever the cause of a problem. The pressure switch is connected to the draft inducer with a rubber hose. The hose can get clogged with moisture/dirt. Take it off and make sure it's clean. The end of the hose away from the pressure switch usually goes to a slide on nipple. There is tiny hole.... the size of your hair.... down inside the nipple. That hole plugs easily and must be thoroughly cleaned. You can use a small drill bit, piece of wire, pin, etc. It must pass thru the hole completely for proper operation. Blowing into a pressure switch hard can damage it. Technically a pressure switch runs on a vacuum so draw slightly on the hose and listen for the faint click.

A clogged or partially clogged exhaust line can also cause the pressure switch to not prove. Animals, insects and kids have been known to stick things in the PVC flue line. Not something to be left out.

A "high limit" is another very common problem. It's usually caused by lack of airflow thru the heat exchanger. This can be caused by a dirty filter, closed down registers, blocked returns, duct clogged A/C coil and other items that inhibit airflow. Check the obvious first and then we can discuss other problems in your thread.

Please feel free to start a thread for your problem appliance and we can troubleshoot further.