I have an older weatherking furnace the inducer motor starts ,
Then the igniter lights then the gas valve opens . now this is where it really becomes technical the flame ignites for approx 15 sec and goes out. thought flame sensor or circuit board replaced both of them and it still does the same thing. also checked the wiring connections through all the plugs. I am an experienced hvac tech but am lost here anyone have any advice
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There is really not enough information here to go on. If this furnace has a thermocoupling, the pilot will need to be lit manually. Follow the manufacturers lighting instructions carefully. If it still will not light you may have a dirty pilot assembly or a defective gas valve. Modern furnaces do not use a thermocoupling. There is no standing pilot light. Each time the furnace needs to be lit it will be done electronically. Many reasons why it wont light in this case. Safety control could be preventing it from lighting for safety reasons. A bad ignitor, ignition module, circuit board, gas valve, pressure switch, gas supply, induced draft motor, blocked or restricted flue pipe, cracked heat exchanger, dirty flame sensor, ect. ect. ect.
power to gas valve ok .. Replace gas valve after checking gas valve coil for continuity/resistance... with valve removed ...and power to valve ... try blowing thru valve from supply side ... stuck valve.... replace...
Without a pilot flame sensor signal the pilot light will not stay on and the heater won't work. Try Replacing the pilot flame sensor, the pilot flame burns on to it and it connects to the valve. It looks like a wire.
I've got an easy way for you to check, and then we can help you from there. First pop open both lids to your furnace (this may require a screwdriver). Turn up your thermostat, push the safety interlock button in if it has one (it's a button that turns off the power when the doors are removed), and watch.
On most modern furnaces (past 20-25 years) you'll hear/see: 1. Inducer fan 2. orange glow (ignitor) 3. Gas valve opening and ignition (flames) 4. Main Fan.
that's normal heater operation. The most common issue here is a burned out ignitor, in which case you wouldn't see the orange glow but the sequence would attempt to continue anyway (the gas valve will open and hiss gas, but it won't lite, then it'll give up and try again a few minutes later).
On older furnaces there's a pilot light to start with, the click of the gas valve and boom... ignition.
On new furnaces it's Inducer fan gas valve opening spark - ignition Main fan.
and there are some oddballs in between, particularly the Lennox Pulse. If that's your furnace just back away slowly and call a professional... some guys won't even touch those, so be sure to mention what it is... if you call a pro.
So go ahead and watch it run, then tell me/us what you see.
If yours has a Kanthal spark/sensor it may need to be cleaned up. The ignitor lights the heater and then it has to confirm the flame is there. If the tip has junk on it then it might light the flame but cannot confirm ignition.
The gas valve solenoid coils work when cool that's why the resistance reading is within the limit but fail when heated. Bypass the thermostat on the burner funnel and if the flame still goes out too soon, the coils are weak and both must be replaced. The resistance reading is reliable only when the dryer does not heat at all or shall we say the flame does not go on at all and the purpose is to check the continuity of the coil.