IF the vent CO was 1400ppm, you have a crack or very very dirty burner. The end result is if the rollout sitch is tripping it means the flame is not getting through the heat exchanger as designed.
There are only 2 reasons why....exchanger is cracked or plugged. Either one requires a lot of work.
Being that particular brand and model the odds are that the heat exchanger is indeed cracked. There are pressed rings at compressed sections of the exchanger that popped and cracks originated there.
A good burning furnace is under 100ppm CO..bet if you take a reading before the circulation blower cuts on it is lower and then climbs after blower start up.
With my experience with this unit and the symptoms described.....you do have a racked exchanger.
Just because there is a crack does not mean CO in house or possible sickness or death. Look at a fireplace....it is a heat exchanger missing an entire side, but there is no danger as long as it drafts. The draft fan is pulling the c.o. through and out.
The only really dangerous furnace was a Lennox Pulse because it ran with a pressureized heat exchanger and it could force CO through any cracks.
Most Tempstar, Heil, Intertherm (all ICP Products) had this issue. It is just when the crack gets large enough that the blower gets into the exchanger and pushes flame.
As long as the CO gets outside your detector will not go off. Also it has to do with clean burn. What is a gas stove or oven? No chimney pipe and burning gas...no detector goes off.....Because it burns clean with minmal CO.
I know this furnace well.
I know the odds are very very high that if it does not have a ringlet crack it will.
I know you are in a quandry of it is cracked or not...but that is what this furnace was known for. If the exhaust showed that high of CO and being on a Tempstart NUGM, it is certain you have a crack.
If you would like to pay a company to disassemble the unit and show you the cracks, I understand. But I have taken these apart, I have replaced the heat exchangers and I do know the historty of this unit. In my opinion if you pay anymore money to inspect you are wasting time and money.
The heat exchanger will be replaced with the parts free but labor not.
100,000 btu would be better suited for your home.
The Bryant and Trane are both great choices with convenient numerous contractors.
Here is a link for you to find a Tempstar dealer for assistance.