These gas furnaces are very simple with few moving parts. Manufacturers added another moving part, and another way for unit to fail, by adding an electrical safety switch, which interrupts electrical command to unit to operate if the switch is in the open position such as when the lower panel is removed. But sometimes the lower panel, even when in place, may not depress switch enough to keep it closed. To see if an open switch is your issue, first verify that key unit components are working -- themostat works properly, gas supply to unit, electricity to unit, gas valve opens, flame ignitor comes on, fan comes on. If yes to all, check to see if the lower panel electrical safety switch works, and is positioned correctly. If the switch, or panel, is out of place or removed, the switch should remain in open position, and the unit will not run. If panel is not pressing against switch sufficiently, switch may not close when panel is in place. Given the coefficient of expansion of the steel plate the panel is made of, the panel may cool and contract enough to hold the switch closed when the unit first turns on. As the inside of the furnace heats up, the panel and frame of the unit expand as the metal parts heat up, and as the panel moves away from the switch due to expansion, the switch opens and the unit cuts off even though the lower panel is in place. This may take several minutes, or longer. If the panel remains far enough away from the switch after the switch opens and unit stops operating, the switch could remain open and prevent power flow/themostat signal prompts from activating the unit even when the unit frame and panels cool and contract. To check to see if an open switch may be your problem, turn off power to unit, remove top panel, remove bottom panel, check switch to see that it physically opens and closes, replace lower panel, replace top panel, turn power back on. Do this repositioning lower panel exactly as you was before you removed it, without pushing the lower panel in toward switch. If unit does not activate, or stops working after several minutes while thermostat is set to command unit to heat, an open switch may be the issue. To test for this, apply light physical pressure to lower panel, toward unit, over switch location. Cycle unit power switch off, then on. If unit activates and runs normally, insufficient panel pressure against switch, causing an open switch, may be source of problem. Possible fixes include putting lower panel back into place repositioned so that it presses more against switch, or taping a very thin shim (such as a square of cardboard cut from a cereal box, but must be done competently as tape and cardboard fluttering loose inside the blower compartment may lead to fan fouling issues) to the inside of the panel so that it depresses the switch more, or bending the panel so that it protrudes slightly toward switch (not recommended as once bent, panel is hard to get back into place properly, and bending may cause other issues), or carefully repositioning or nudging the switch toward panel (its fasteners may have come loose inside the unit frame, or switch may have vibrated out of place over time). If unit still will not cycle on, use a multimeter to see if the switch, when closed, allows current through the line (a continuity test). This might detect a bad switch. If the switch tests good, do a continuity test when the panel is in place, using alligator clip extension wires to test to see if the switch closes when panel is in place. Do this this test with main unit power turned off. If switch tests good when panel is removed while you are physically holding the switch closed, but tests open when panel is in place, manipulate/reposition/shim the lower panel as noted above.