Furnace help.

Wondering if any body can fill me in.
I think my Gas Furnace has somehow inherited some strange ways.
Just recently I seem to have noted a difference in its operation.
here's whats going on. (blowing a lot of cold air)
-stat calls for heat -fan comes on -in viewing through the inspection window, pilot light is on. -about 5 seconds after fan starts is tries to light a bigger fire and glows red for a couple seconds then dwindles back down to just the pilot light. -fan continues to run for a minute or so, blowing cold air. then goes off. - about every 3rd or 4th attempt, the fire does get going and I have heat.
Seems to me that the fan runing right away is not allowing the pilot light to ignite any of t he gas.
Its one of those things that you don't pay attention to it while it is working ok. So now that I think there is a difference I don't really know how it ran before and if there really has been a change.
Thanks
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Thermocouple ?
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snipped-for-privacy@yahoo.com wrote:

No.
hvacrmedic
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Mrs. Beezley wrote:

There are several possible causes of burner short-cycling. You should have the unit serviced and explain to the tech what you have observed. The list of possibilities will vary depending upon the type of ignition/flame sensing system that you have. Depending upon actual cause mild explosions of raw spewed gas can occur. Get it checked out by a professional. Odds are that it's something as simple as a weak pilot on a 3-wire sensor, or dirty flame sensor on a spark ignition system, but it's impossible to say from here. Other possibilities can be life threatening.
hvacrmedic
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Mrs. Beezley wrote:

I was thinking of creating website on "modern" gas furnace repair....
But here is the short list...
1) Stat calls for heat 2) draft motor pulls vacc. 3) you should hear click from vacc brak. 4) ignition 5) gas valve opens up, listen for sound of gas 6) observe flame, blower motor turns on. 7) flame sensor detect flame 8) if flame is not detected the valve will shut off furnace in less < 10 sec 9) next *if* there is a problem limit switches will break the circuit.
How can you tell problem? Everything in the furnace is powered by 24v. By checking 24v on both sides of the limit switch, or vacc sens you can tell if it is open (24v on one side), indicates a problem...
The fact that limit switch opens does not mean it is bad. It could indicate problem, blocked return filter, etc.
Check wiring, burnt wiring should be replaced.
Most furnaces also have led, that would flash like two long and 3 short code 23.
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Flame sensor is most likely bad
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