need help with lennox unit

[fr420]

M# G24M4/5-120A-2

Unit Fires normal that shuts down after about 20-30 seconds later. It just keeps doing this, doesn't go off on any faults. Checked all safeties, they are not opening. I belief I would get fault if it was the flame rod, but not sure.

[cmajerus]

does it do it with the thermostat jumpered out?

[fr420]

I checked voltage at terminal strip to make sure still calling when it shut off, so tstat is not the issue. Thanks though

[cmajerus]

ok, does it have the intermittent spark pilot, I have had the flame rod do this when the main burner comes on it pulls the pilot off of the sensor and never locks out just keeps doing what you describe. If you can't get the orfice and hood clean enough, then replace the entire pilot assy.

[ascj]

I would double check to see if the pressure switch is dropping out. Watch it closely has it will open then close real fast. Keep your meter off auto range. Being a -2 G24, it might be the inducer. http://tech.lennoxintl.com/PDFs/9510.pdf

[fr420]

Little more info....while I was checking this out I put my meter on every safety to see if anything was taking it out. Like i said this was happenning every 30 sec, so I just went from one safety to next. I beleif it is gas valve or board, because if it was flame sensor I would get a fault , right? I had my meter on gas valve while it was running and notice that gas solonoid drop out before voltage dropped. I did this by listening for solonoid and watching meter, but It was only a split second later that voltage dropped. Don't know if I got a bad gas valve or is that voltage residual. As of right now, I plan on replacing gas valve and flame sensor.