Hey Jason,
check to see if the vent or intake has closed off do to the condensate. I assume the tank is a mid eff. and has been side walled? With the sudden drop in temperature over the last week, it may have chocked off the intake or vent. If they are clear, probably the thermocouple. LMK, if I can help.

What do you mean by side walled?
edit - This has happened last year when it was really cold.

if it is a power vented unit then there will be 1 or 2 pressure switches that determine if the vent is venting properly, one of the most common problems is a blocked condensate drain that allows water to backup in the pipe causing a blockage in the vent which causes the unit to shut down, in this case, the unit ussually starts briefly and then shuts down

Jason,
Give me a call, I'm at home right now with a sick kid but can probably walk you through it.

I found the parts list and I need some help identifying the flame sensor.
I believe it is part# 25 called fv sensor.
http://i379.photobucket.com/albums/o...hgpdmanual.jpg
http://i379.photobucket.com/albums/o...gpdmanual2.jpg

does the hot surface igniter glow red hot while the unit is trying to ignite?
the FV sensor is a Flamable Vapour sensor, it will lockout the unit if vapours are present, gas vapours, paint vapours, etc.

When I turn on the tank the fan turns on, glow plug heats up, burner ignites for 5-10 min. and then it shuts off. It cycles twice then turns off and goes into vacation mode.

is the fv sensor the metal rod that has to be cleaned?

Now I'm confused, my hot water heater is working again.
Seems to be related to cold weather because it was -30 when it stopped and today it's -18 and its working .

no, the FV sensor senses the pressence of flammable vapours, a flame sensor is a metal rod that will produce a voltage when heated, detecting a flame

I would suspect a problem with the vent or the pessure switch for the vent, extreme cold and or high winds could cause an abnormal pressure.
The pressure switch is # 27