funcky

Im working on a friends trane gas furnace.
Unit stopped firing up few days ago...will work occasionally. I told him to power unit down for about half hour and try it again. He said it came back on, but didnt satisfy T-Stat before it went off.
I showed up on 2nd night, unit came on, lasted about 10-15 minutes then shut off without satisfying t-stat. It tried to start through usual cycle of 3 times then shut off totally and gave a diagnostic light blink showing external lock out...too many tries. All limit switches are getting 24 v...there and back. When unit cut out after running for 10-15 minutes, blower motor stayed on. Manual states when this happens means return air is clogged. Removed filter and tried again...this time did'nt start at all.
Did check amperage of glow plug...getting 3.5 at highest ...door panels states ign. should get 6a @ 120 v. ! Now, glow plug lights up, getting 3.5 amps, but gas valve never opens.
Flame is blue and solid.
The circuit board is not letting the valve open, yet I cant figure out why it lets it sometimes.
does anybody have any diagnosing ideas.
Im figuring circuit board or valve problem although the unit will occasionally
fire up, it does turn off after about 10 minutes.
Could it be a heat sensor turning it off? Not much of a diagnostic on board. Just keeps saying, external lock-out due to too many start up attempts.
If anyone has any advice, please...I am a tech!
Just called customer hour and half after i left...unit has been off since I left. wife stated unit came on ...is there a heat issue turning unit off before t-stat is satisfied. Why does blower stay on after flame out...stays on for a while....minutes not seconds.

A Smarter Home

I would look at, inducer vacuum switch and all the lines to make sure they are not clogged

"glow plug lights up, getting 3.5 amps, but gas valve never opens."

Dr Heat

board
I assume you have checked the high limit.
Be shure vents are open returns clear
check the Evap coil to see if it is clean

MechAcc

Check voltage at control board W - C. We had one this winter that the thermostat dropped the voltage when the control board energized the gas valve. In our case the valve would chatter.

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cleveman

I had a similar problem this week.

Furnace would run for 10 minutes, then shut down. Occupant would play with thermostat after awhile, then it would eventually start up again, start from beginning.

I made sure inlet and exhaust were clear, then just disconnected inlet inside. Unit would run for 45 minutes with combustion door off.

Had tech come and I took off the fan door for him to look around and just before tech shows up, I see an old filter stuck in front of the fan. Embarassing. So we figure that is it.

Later in evening, occupant calls, same problem. I tell them to remove the combustion door and come the next morning with tech again.

Tech has checked all amps, drain hoses, etc. We are stupified, so tech takes off exhaust blower and finds the gasket has failed and is hanging down into the exhaust hole (only about 1 1/2 diameter). Replaces without gasket and furnace won't ignite. Makes gasket with high temp silicon, replaces and furnace runs fine. Slight noise which was present before is no longer there. Exhaust blower runs cooler.

We thought it was getting hot and shutting off because of the additional filter which had come off the rack and lodged by the fan. We thought that was why it would run with the combustion door off, because this allowed enough airflow to cool it down.

But it was the failed gasket which was letting heat escape which was the culprit.

Willis15

If you have a magnahelic, check your static pressure. Your return or supply duct work could possibly be undersized. Check this after you've proven that your rollout out switch and high limit aren't locking unit out.

trptman

Had a furnace once that was operating erratically similar to what yours is.
It turned out to be that the exhaust vent had a partial blockage. Just enough to SOMETIMES not allow enough vacuum from the vent blower to the pressure switch.
removed the partial blockage and it's worked fine (knock on wood) for a couple of months now.
You might check that out.

airwolf

Try Flame sensor, clean it if it works replace it