What model is this? A high or mid efficiency?
I have never worked with a Janitrol Goodman, but here goes.
Most of these furnaces work in the same manner.
The thermostat calls for heat and turns on the exhaust ventor blower via a relay. This ventor will keep blowing as long as the thermostat is calling for heat.
Then a diaphragm switch closes from the air pressure created by the ventor fan. You may have a plugged condensate drain. Water backing-up in the exhaust therefore not enough draft for the diaphragm to close. Look on the wiring diagram of your furnace on the furnace door. Check to see if it's 24 volts. Set your volt meter for 30 VAC. Put probes on the two terminials of the pressure switch. Zero voltage means pressure switch closed when ventor fan ruuning (OK). Voltage detected then switch open (Vent pluggged or condensate drain plugged or bad diaphragm switch).
What reset do you have to hit? Sounds like the condensate drain line from the furnace is plugged and not enough draft. The Flame roll-out switch?
Go to www.furnaceparts.com I see Janitrol used a B1401011 flame sensor & low voltage ignitor PGB early serial #9607. or Hot surface ignitor B14010-18S on the GMP, GMV, & GMPN models. It sounds like you got a flame sensor.
Anyway perhaps you can get a manual from the above webb site also.
Check the exhaust vent and the condensate first.
If uncertain of doing this repair please call someone that is.