Thanks for your reply. I am an HVAC commercial tech and not very familiar with these MH furnaces and how the exchangers are built. It looks like a barrel to me. Can the exchanger be accessed from the front to visually inspect the HX. I will grab the combustion analyzer the next time I am at the shop...

Have a Coleman DGAT056BDC (SN starts with 95) that is having issues with going down on flame failure. It will light then go off within about a minute or so, then relight. The HW S86 controller will allow 3 flame failures then lock out which is happening. Had this similar problem a couple of seasons ...