I have a Trane xe 90 and sometimes it heats and sometimes it doesn't. I actually haven't looked at it yet but have been given the details on the unit so I don't know if it's a HSI or an Ignition module. I'm thinking it might be a pressure switch but I'm not sure. What do you think?

I think you need to look at it,, as it goes through its normal start up steps,figure out which/what step is not working

I checked out the unit last night, air filter is good, no clogs in the exhaust vent line and after I re-set it, it worked fine. I asked my friend next time it's not working but the fan is on to check the fault code and call me to tell me how many times it's flashing then I'll narrow it down. Thanks