duo therm furnace won't stay lit

When you have it out on bench it fires and sustains main flame for how long?
When installed....how long is main flame staying lit?

IS this a DSI model or pilot model?
IF DSI....is the 12V DC correct polarity...gas valve won't stay open if polarity reversed. Also have to have good clean tight grounds on furnace

Are the vents installed correctly....the 'coned vent' is flue outlet.

Not sure about the high limit, didn't bypass it, but when the furnace is turned on, the failure is quick, I don't think, long enough for the high limit to open.

I blew out the gas line, replaced the regulator, problem persisted, ran a direct line from a separate source still had problem. Have taken the entire unit apart, all chambers clear. I thought maybe the squirrel cage might be out of alignment causing air to blow back, or perhaps something not sealed properly???????

Have you confirmed that you have gas flow to the furnace?

An oil trap in line to furnace can block gas flow.

Post the Model number of the furnace please. 1987---That year and around that Duotherm produced the 9000 series and took it OFF the market after a few years, because of intermittant staying lit. It was due to the CASE PRESSURE. A symptom was as you described, Works on the bench AND if you remove or slightly remove the case cover when installed, it lights and stays lit. IF YOU HAVE THIS MODEL, A REPUTABLE SERVICE COMPANY WILL NOT ATTEMPT REPAIR. It would be a waste of time and just cost the customer money. They CANNOT be repaired and be guaranteed to work. There are very few parts and Dometic has not supplied parts for over 20 years. The only sure fix is to replace with a compatible model that will fit in the space. Dometic used to send out NEW units under warranty and they sometimes failed to stay lit. Doug

That sound like you are cycling on the high limit switch.....

Runs fine on bench but on/off when installed...air blowing back

You need to make sure combustion/exhaust chamber is free, clean & clear of any obstructions. Overheating in combustion chamber due to inadequate air flow thru it and high limit switch opens shutting down burner.

High limit switch is in same circuit as sail switch....so when you jumpered the sail switch the high limit switch was still in the circuit going to control board

not sure where the sensors are that you are referencing

intake and exhaust are clear

are the tubes that connect to the furnace when installed in the TT clear or do they have wasp nest in them.

Sensors may be bad or not attached to the proper location. Or bad ground.