SF42 Furnace Running - will Heat for a few minutes then cold

[BengalBus]

Our furnace is running/blowing- will blow hot air for a few minutes then turns to cold air.  Propane is working - can light stove burners no issue.  Bypassed sail switch and same issue.  Anyone experience this?  We are suppose to head down South tomorrow from Nova Scotia- but obviously will have to boondock at least one night on the Northeast Seaboard- we're running down the 95- and heat is imperative!  Any suggestions welcomed!

[BengalBus]

FINAL UPDATE- at least until the next issue LOL- New Thermocoupler resolved the issue- we have HEAT!  Now we will make our way SOUTH where we hope to not need it- or very little of it anyway.  This was my first time using the forum in the Good Sam Community and I must say all your responses were great,  gave us lots of things to trouble shoot, and I appreciate each and everyone of your responses!  I wish you all a Merry Christmas and Safe Travels if you are out on the road!

[BengalBus]

UPDATE- we are headed to an RV store next Province over to pick up new thermocoupler and Control Board- we think it's the coupler now- but if it's not it would have to be the board- so I may have a control board for sale as they won't accept electronic returns!  I appreciate ALL of your responses and will keep you tuned into whether we are successful!

[StirCrazy]

how old is the furnace?  might be worth just replacing them both or keeping the control board as a back up.

[BengalBus]

It's a 2009 Triple E INvitation Class A- so it would be 15 years old- yes we'll likely keep the board as a back up!  

[bid_time]

Just for future information - you just very lightly sand the thermocouple with fine emery cloth to clean it. You can still do that and you’ll have a spare on hand if you ever need it.

[StirCrazy]

doesnt always work, sometimes they are just old enough that the magic leaks out of them haha

[way2roll]

have you tried simply resetting it? I had your scenario happen once. Turned it off at the Tstat, or pulled the fuse (can't remember which) , waited a minute, turned it back on and it worked fine. 

[BengalBus]

Yes- we have- he has just taken it all apart- tested the temperature coupler- it's fine- so next two things are the propane regulator and the control modluler- not sure if those can be tested!

[joebedford]

The one time this happened to me, it was low 12V power.

[BengalBus]

We checked that yesterday- it was good!

[StirCrazy]

so let me try get this clear, the thermostat calls for heat, the furnace does its purge cycle then lights, runs for a bit and goes cold.  

1, is it actualy lighting?  are you getting hot air then it shuts off?  or is it trying to light then failing?  

I had the issue where it would light, sometimes, but when it shutoff and went to relight the next time the thermostat called for heat it wouldn't light..   then eventualy it wouldn't light at all.  this ended up being bugs in the exhaust path and even inside the main burner.  once I took it all apart and cleaned it it worked perfect untill I lost a board.  with this it wouln't light at all.  would go through the purge cycle but just wouldnt light.  

[BengalBus]

Yes- it's lighting and will run for maybe 5 minutes max at hot air- sometimes it shuts off and then comes on with cold air- othertimes the hot air just transitions to cold air.  He's out tearing it completely apart now- we have access to a SF35 that we could get parts from if needed- from what we are reading the cores are identical not sure about the thermacouplers and modules though.......

[bid_time]

Clean the “proofer” rod. It’s a small thermocouple above the main burner. It tells the furnace “brain” that the main burner lit and all is well. Otherwise the “brain” thinks that raw, unlit gas is pouring into the camper and closes the gas valve.

[BengalBus]

Seems this thermocoupler may be the issue- how do you clean it?  We've tried sourcing it new but all the parts depts in our area are closed for winter- we do have access to a SF 35 we could rob off of as well if it's the same......