Light on solid, ventor on, ignition, no fan, flame goes out after 30 sec, ignitor heats up, no ignition, no fan, recycles. Fan works w/fan sw. on stat. Cleaned flame sensor. Same result. Will sometimes work continuously w/fan switch to on and stat in heat mode.

SOunds like a bad fan relay. But if that was the case it should work with the fan running all the time. Inless the heat exchange is cracked

Relay runs fan w/fan sw to "on". CO2 detector shows no gases from cracked exchanger. If I put fan sw to "on", then unit runs fine, but continuously. Will not stay lit in fan "auto".

The furnace is going off on high limit, I would start by taking a apm draw on motor while it runs with t state in on posistion if normal (2-3.5 amps) try the same test with t stat in auto, if reading is around 6-12+ amps replace motor and capaciter. If no reading check for power on control bored at the heat terminal upper right hand corner, if 120v is not read while burners are light and t stat is in the auto setting I would replace control bored