No flame, at a loss

Ryan7754

I have a Goodman natural gas furnace
AMS81005CXAC. Over the summer the AC went out, bad board no power going to blower motor. Ordered one, DIY swap, cold air works. Didn’t check furnace after board swap (amateur). Told myself I would check it after the house got below satan’s armpit levels. 

Forgot and life moved on for about 2.5 months when we tried to turn the heat on. 

induction motor turns and igniter glows red hot. There isn’t 24v going to the gas valve to kick it open. Interesting thing is when I test on wires unhooked I’m seeing about 2v, back stabbing on wires while hooked to gas valve seeing sub 1v. 

Thought I got a bad board, ordered and swapped again, same result. Board ordered both times was a Goodman Amana Emerson Furnace Control Circuit Board 50M56-281-02 50M56-291 
Roll out switch’s both test as having continuity 

Any suggestions?

HomerJSmith

Is the thermostat switched to heating mode? Gas valve or supply turned on?

when I test on wires unhooked I’m seeing about 2v, back stabbing on wires while hooked to gas valve seeing sub 1v

Please explain what you mean.

pedmec

How many flashes on the control board? Check the auxiliary limit for continuity.

Jamie Hall

2 volts relative to what? Each other? Check everything in the safety and limit controls -- you are not getting power to the gas valve.

HomerJSmith

Gas valve on/off switch in on position? Check this service manual https://hvacdirect.com/hvac/pdf/GDS8-Service.pdf

Ryan7754

Thermostat is turned to heat 

Board goes to single flash after 3 attempts to fire. 

Gas valve is turned on

checked six high limit and it has continuity. Paper work states it should normally be closed so good there.

2 volts measured at gas valve on the brown wire hitting the gas valve. Measures at 2 volts when unhooked from gas valve and both wires going direct to volt meter (gas and gas common wire’s) If wires to trigger valve are hooked up to valve and I back stab them it’s a sub voltage reading

Really stuck on this one. Know I’m not getting voltage but it appears all safety devices are reporting correctly but still no power going to valve. 

Ryan7754

GOT IT FIXED!! just wanted to update incase someone in the future runs into something similar. It was the auxiliary limit. I checked it for continuity and it was good, once voltage was applied it wouldn't pass it though.
Many thanks to everyone that assisted, now to call the HVAC company I paid to come out here and tell me it was the board.

Also anyone know what temp range Aux limit I need to use?

109A_5

Hello @Ryan7754,

None labeled "Auxiliary" in this parts document but maybe you can find the temperature you want (some listed) or just the factory part number for replacement.

https://hvacdirect.com/hvac/pdf/GDS8-Parts.pdf

HVACNUT

Where is the auxiliary limit located, in the blower compartment? 
I don't think I've ever come across a N.C. limit that mysteriously opened once voltage was applied. 
The ratings of the limit should be written right on it. Small lettering but it's there.