Fan runs and u can see the glow plug is lighting but it does not fire up and only runs for a few seconds

Is your inducer fan coming on first? I know on a gas heat pump yu will hear the fan/inducer turn on for a few seconds, then the igniter will lite, then yu will hear it fire up, this is a safety feature sequence, no gas movement thru system, no ignition, also check the exhaust pipes for holes or cracks

Here is a useful reference:
http://www.desatech.com/techDocs/DESA%20SPARK%20PLUG%20IGNITION%20SERVICE%20MANUAL.pdf

I believe there is a thermocouple that must be malfunctioning.

The glow plug light comes on to show that the glow plugs are warming up. You should not start the vehicle until the light turns off, as it can damage your glow plugs. The light should turn off after a few seconds while the ignition switch is in the on position. In short the vehicle is working properly.

Sounds like it`s ready for the glow plugs replacing, with it trying to fire it sugests one or two glow plugs are working but thats not enough with the recent drop in temperatures we have seen.

this have a flame controler, if the flame didn't goes on in few seconds the unit automatically stops. back plug and start again, if not star after tree times check the flame control thermocuple, or get the unit in service to fix it.

Check fuses to see if there are busted ones. Then check heater plugs if good. Heater relay included in check. Engine wont fire if the air inside the chamber is cold. Then check fan belts for wear, replace if necessary.

The Miller furnaces sometimes will do that. Does this have surface ignitor that glows. Kill power to furnace for few seconds and turn it back on. See if it will go thri the cycle and see if coil glows. If not chances are coil is bad. Need to call service tech for free estimate. Rus

Hi Whitneyford
Your priblem sounds more like the glow plug timer relay than the exhaust. White smoke after start up is a sign of diesel entering the exhaust before it has burned completly. This is as a result of the glow plugs switching off too early, which is usually caused by one or more faulty glow plugs or a faulty timer.With engine COLD, IGNITION ON... Check voltage with all 4 plugs connected. You should have a reading of 10--12 volts
If there is no power, check in-line fuse on loom coming to plugs.
If power is present, check for How Long. If only for a few seconds, the problem is with the timer. This unit (the larger of the relays and usually black in colour) is fitted on the fuse board, and there are a few different types. They come in 7 pin..8 pin..and even 9 pin units.
The glow plug light is often NOT accurate (even with a new timer unit fitted) as to the time of heating, and you will almost certainly find that power will flow to the plugs for quiet some time after the light goes out. This is not unusuall with The Jetta Tdi.
Good luck.
I hope this helps you sort out this problem
John

HI, there are a set of coils on the gas valve assembly that have gone bad. these coils are responsible for the opening and closing of the gas valves. your coil assembly is damaged, in return, causing this interruption.

simply replacing the coils will fix this issue.

Check the glow plug thermo sensor...found in the cylinder head.
Unlightly anything wrong with injector pump.
Good luck
John