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I've got a problem with my 15 year old Goodman furnace. It starts w/o problem, inductor begins working, hot surface ignitor lights up and ignites the gas. Then the main blower starts working and the whole thing stays on for 2-3 minutes. Then the flames go off before the thermostat reaches the target temperature. Though it continues to recycle the air. It will restart 20-30 min later with the same effect. Sometimes the flames don't appear at all (I guess gas valve stays off) even though everything else stays on (including HSI).

I checked the termocouple. It looks clean but may be I still should replace it? Could it be a faulty air pressure switch?

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Are there any diagnostic lights/displays on the furnace? What is the model number? How long does the blower run after the gas shuts off? Does the furnace ever lock out (you have to cycle the power and/or thermostat to get it to come back on)? –  Tester101 Nov 20 '13 at 3:41
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There are a couple of things that it might be: It could be the flame sensor. I have seen them where they look clean but are dirty enough that the DC voltage being generated is not enough for the circuit board to keep the gas valve open. I would pull it out and clean it with some scotch brite or a brillow pad. be care full not to damage the porcelain base. It is generally fairly easy to remove, one or maybe two 1/4 screws holding it in place. Gently pull it out and clean it with the pad like you would if you where sanding a wooden dowel. The 2nd thing it might be (less likely) could be a roll out switch. If your air filter is dirty it would prevent enough air flow across the heat exchanger to keep it from over heating causing the roll out safety switch to open. If this is neglected for a long period of time it can cause the ROS to weaken and begin to trip at lower than designed temperature. ps. there are typically LED lights on a the circuit board that will blink error codes to help in diagnostics. It is best to capture that info and will usually point you in the right direction.

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