Fi have a older furnace that operates like this. main burners will come on then the blower turns on and furnace continues to heat untill it shuts down on the high limit. there is no blockage on the cold air. i recently changed the fan and limit control. i have no way of knowing which wire was the load wire from the blower. if i reversed these two wires coming from the fan could it possibly reverse the rotation of the motor? i am not sure if the furnace is suppose to heat up until it shuts down on the high limit. this sequence is with thermostat up all the way, calling for heat. thank you, alla

power to gas valve ok .. Replace gas valve after checking gas valve coil for continuity/resistance... with valve removed ...and power to valve ... try blowing thru valve from supply side ... stuck valve.... replace...

Furnace burner and heat element are not the same. I think the fan time delay, a sensor that turns the fan on when this sensor senses heat, is shot and needs replacing, for a furnace type heat ( gas heat)

Check out the flame sensor see if it is clean and check the connections .

I would be looking at the heat exchanger. They are designed to have the heat from burnt fuel flow thru them. Cracking or pin holes will let the super heated air into the furnace interior and in there you have some over heating protection some look like buttons, older they look like an external thermostat, and these get the hot gas from the burners and trip then the furnace shuts down. When the furnace comes on again some times the blower will come right on because the sensor says hey this thing is hot when in fact it isn't. This also can and if bad enough will cause carbon monoxide poisoning, please inspect this closely or have a professional look at this. There is no price ever to great to replace a human life.Sorry probably not the answer you wanted but better safe than sorry!

its the fan control AFC not shutting off , it comes on at temp rise say 150f drops back to 130f with fan on , burner shuts off fan runs untill temp falls to 100f the fan shuts off in this case it keeps running

Sounds like fan limit switch/fan timer

This will cause the main burner to run untill the over tempreture switch is tripped therefore shuting gas supply and running fan on high till over heat condition is resolved then it will restart burner and repeat
continuing to run heater like this will cause heat exchanger split.

Check the board for a set of dip switches. Check against the old board for switch positioning. These control fan on/off times. Ensure you turn power off before changing switch positions. If no dip switches,??

Yes. If the high limit is tripped, the furnace will bring on the fan continuously until the limit resets. Often, they will not reset if tripped repeatedly. If the furnace is too large for your application, you probably are not able to move enough air to keep the heat exchanger cool enough.

I am providing some information, but I encourage you to have a furnace technician look at your furnace. The reason is; the limit switch trips when there is a problem that may present a fire or combustion hazard. I've seen these switches go bad, but not very often. Usually they are tripping because the plenum is sooted up and excessive heat builds in the combustion chamber. This is common in older furnaces that have pilot flames- the flame runs all year and soot deposits in the plenum, plugging the air flow through the combustion chamber after years of use.

The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs adjusting.

To adjust the switch, remove the control's cover. Under it is a toothed dial with one side marked LIMIT; don't touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit and the lower one at about 90 degrees Fahrenheit.

Sounds like mine did
it was a realy switch