ICP N9MP2100F14C2 Furnace Overheating

Ok, so my furnace for the upstairs of the house has been acting out the last few weeks. We only moved in in October so don't really know much about the set up, but 2 HE furnaces, no idea on the maintenance history and the main floor one runs very smooth. Its an ICP N9MP2100F14C2 furnace that is around 10 years old.

Anyway, upstairs one is overheating very quickly, will fire up and run for 1 minute or so then the limit switch shuts her down, I get 4 flashes on the Board (Limit switch open) then 30 seconds later it resets and off it goes again, it does eventually reach the temp that the thermostat is calling for but only after running numerous short cycles.

Here is what I have thrown at it so far (in my normal fashion), starting with cheapest first:

- New air filter (not a super expensive one which I find tend to restrict airflow)
- New flame sensor
- New ignitor
- New limit switch
- 2 x new roll out switches
- New pressure switch
- New thermostat
- New main control board
- New draft inducer blower

Now I can feel that the furnace is definitely overheating the main vent up to the air outlets upstairs, just above the furnace is very hot to the touch and the vent pipe is warmer on this one too (when compared to the other one that works fine)

I have checked the venting for blockages, nothing there, checked that the air intakes upstairs are all uncovered and even tried running with the bottom door off/no filter to increase the airflow.

Here are my last 2 options for what I think is wrong:
- damaged heat exchanger
- Fan motor on main fan at bottom, but this does run and no funny noises or funny smells from it

Anyone else have any ideas before a reach my last resort and call out a tech :wink2:

I know I could have had one out and probably had this fixed for less than the all the parts I have bought so far but hey, I'm self isolating and what else is a man supposed to do...