Takagi Jr Radiant Heat Issue

Help! My one-year old Takagi Jr used in a radiant heat installation has died. It worked fine last winter and seemed to be Ok this year until we got some below 0 weather. If I turn off elec., wait a minute, then restart, it will fire and everything seems normal. It will run about 1-2 minutes, then quits and shows error codes. Error codes seem to alternate between light flashing 3 times and light flashing 4 times. I've cleaned the flame sensor as mentioned in another post (and damaged the fiber gasket but I think i pieced it back together OK). The only difference between this year and last is that I added a direct vent. I bypassed the direct vent (opened the cover on the unit) and then the unit ran until the thermostat shut it off but now it will not run again. I should also mention that the exhaust vent is dirty with what looks like soot. Hope this is not too confusing and someone has some ideas - No one in my area seems to work on these units.

Check for obstructions (including frost) on the air-intake venting for starters.

Is this a propane or gas unit (and is it on the correct fuel?).

Call the Takagi technical help line- they have a reputation for being able to talk most anyone through a debugging cycle.

Thanks for your ideas - Both intake and exhaust are clear. This unit runs on LP and the dip switch is correctly set for LP. I will try Takagi tech support.

IIRC, the dip switch only indicates to the control board which coefficients to use in order to have the same control board for both the -LP and -NG models. If the model number ends in -NG you have it running the wrong fuel (it'll usually run, but with the wrong mixture, and neither it's longevity nor it's efficiency will be near expectations.)

In cold weather frost formation on air intakes can make burners run fuel-rich, and can show up as sooty deposits before it out & out won't run.

According to the manual (see p.17) a 3-blink error code can mean issues detected with any of :

Hi-limit
Igniter
Flame Sensor
Main Gas Valve
Proportional Gas Valve
Abnormal Burning

A 4-blink can mean issues with either:

Fan
Pressure switch

Do you have a Backflow preventer installed in the Exhaust pipe?? if so make sure that it can move freely.I have one on my unit and i found that it had stuck shut my bad should have done the yearly check on the unit I cleaned out the pipe and made sure that it moved freely and it fixed the problem.