Heil furnace model PGF024K060A comes on fine runs to set temp kicks off heat, blower stays on but heat won't come back on until you shut down system

Unit fan runs constantly, exhaust fan runs constantly, gas burner will not ignite. Unit thermostat will not control fans running. I have replaced the thermostat and main control board and it still does the same thing unit still will not shut off except removing main breaker.