Hello, I have a customer who's heat is not working. I approach the furnace and see what I have, I jumped the r to w and everything did as its suppose to do ,when my burners lit it stayed on for about 4 to 8 seconds and shut off now from my knowledge and experience I have had a furnace that did this once before and it was simply the flame sensor rod needed cleaning. I tried to run the system twice, both times main burner shut off so I disconnected the flame sensor rod cleaned it with sand paper and also if that didnt work I was going to test it by disconnectng the the wire from the terminal and placing one lead on terminal and one lead on flame rod wire from my knowledge any direct current above 2,0 is good and below 1.6 is a bad flame rod but after I clean the rod and install it back into the furnace the hot surface igniter lights but burner want light at all, I do have 24 volts coming from ifc,to valve all switches are closed roll out limit switches etc they should because no reason for them to open when there is no fire.
From my knowledge I feel like its no gas pressure or no gas flow the gas valve has a coil ohm reading of 98.0 ohms I think thats about right because from another job I replaced a gas valve and the reading was 60 something and the new one read about 90 some thing or 100 ohms so gas valve I suspect is good. And the customer gas was just cut back on yesterday morning, and another thing that bothers me there is no shut off, drip tap, or union near this unit. So I feel the first 2 times the burners lit was old gas that was still in the line, first I need to see is the shut off open, I dont have a water manometer to check the gas pressure in and out the valve, any suggestions, I know the sequence of operation and all, I am a certified tech but nothing but 4 years experience still come across problen that I have to go back to my book for, I know I am in the ball park because to me heat cycle is very simple, worse case senario ifc is bad which I doubt. Input output is good or gas valve is not functioning correctly but I doubt also , and another thing the blink lights is one blink which mean ignition failure probable causes gas pressure no gas ,gas valve, flame sensor, and if this could be cause by a open limit or roll out or over temp stack let me know but I seriously doubt it because I think these open when burner is lit, let me know pros and service techs.