Furnace wont light
So you wont necessarily catch it open unless you catch it open!
There are tatttle tales to leave on switches that will tell you if it opened overnight etc. I simply bypass the limit temporarily and see if the limit opens with an ohmmeter, the limit will normally never open up, unless defective or a cracked heat exchanger, obstructed filter or AC coil etc.
With these furnaces the components are really cheap wholesale and tend to fail about every 7 yrs or less.
These being the Ignition module either Hot Surface Ignitor,HSI or intermittent spark , the pressure switch for the induced draft which proves the draft exists
its tubing and reaming both orifices where the tube connects with a tiny drill bit,
flame sensors are simply ground rods that do not ever fail in reality as even if the ceramic cracks the flame rod is still ground and senses the DC signal of the pilot flame thus proving a pilot flame exists, the terminals SENSE and GRND are common on the ignition modules of all types as they both use flame rectification to prove a pilot exists.
In other words you can troubleshoot a few 2--50 parts that fail with regularity or do the customer a real service and perform a tier 1 level protocol repair as in Air Force 1 maintenance! It is not the parts shotgun to replace all the weak links that are cheap and not worth overlooking and the customer next month or year has you back for the next part you didn't install the 1st time,
Honeywell and Robert Shaw Grayson make retro fit lits that are about $300
this includes a new gas valve! ignition module!! pilot assembly, essentially a refurbished unit if you toss in the $10 flame roll outs and $30 limit and $20 pressure switch, the only weak item then is the induced draft motor and they are a bit pricey so I don't prophylactic ally service them.