having a problem with a gas furnace, it comes on and runs for about five min. shuts off and will not start?

Question

again.I checked all the wiring and its ok,if you take the front door off the furnace and push in on the safty switch, the unit will start again but after it shuts off it will not start again. can someone tell me what may be wrong with it. the people are getting cold, they have kids and no money for repairs. the furnace is a carrier and its about 4 years old

Answer 1

Hi,
The way you describe what is going on it sounds like it’s the Thermocouple or Flame Sensor, you need to take a look to see which one you have fitted to your furnace.

They both look very similar and they both sense the pilot fame, but work differently. The Thermocouple is made two different metals and when heated it produces D.C. voltage from the energy of the flame. If it starts to break down it can take longer to get heated or stop working all together.

First you can try cleaning the probe, it that doesn’t work you will need to buy a new Thermocouple or Flame Sensor.

If the pilot light goes out and the sensor cools down it shuts off the gas supply. Or if they are not close enough to the flame to remain hot they also won’t allow the gas to come though.
When they start to wear out you can get the problems you are experiencing. Make sure you don't mistake the sensor probe for the ignition electrobe.
Below are some sites for you to have a look at and one site has the part you may need to fix your furnace.


http://home.howstuffworks.com/pilot-light.htm
http://home.howstuffworks.com/how-to-maintain-a-furnace1.htm

Answer 2

From your modest description, your furnace is going into a lock-out condition (i.e. it locks out the ignition sequence because there is some type of problem). By removing the door, you are actually opening the safety switch which removes all power to the furnace. This removal of power causes the ignition control board to “forget” about the problem it had and by you pressing it in re-energizes the furnace and the ignition sequence begins again.

With that said, you want to locate the ignition control board. It will be a printed circuit board with at least 6 (and perhaps as many as twenty) wires going to it. On this board will be an LED and either on the board, blower door or main furnace door will be a troubleshooting chart that corresponds the number of flashes of the LED to the problem.

With the door off, press and hold the safety switch in. Once the furnace goes into ignition, continue to hold the switch in until the furnace locks out again (might be handy to have an extra set of hands to take turns holding the switch in). Once the lockout condition occurs again, the LED should flash a sequence of codes. On Carrier furnaces, there are usually a rapid flash of one through six followed by a slower flash of one through nine. Once the error code is reported – repost what the corresponding error is and we’ll try to help you figure out the problem.

Answer 3

Fancyname may be right. or it could be flame failure and you are only getting through the purge cycle. Either way you need someone to look at it these things can be very dangerous. Perhaps ask a local service club (apex rotary) or a church if they have anyone with expertise (qualified).

Answer 4

Its the thermocoupler get yoru professional in to fix it.