Can you tell me the order in which each componet of a gas furnace comes on to start the furance?

Sounds like you fan limit control is not set properly. Or the blower speed is set to high.

If aquastat wirings are ok, and control module had been set up properly, the the furnace should start.
Check if bypassing the control the furnace starts, if it starts then check aquastat wirings.
If it depends on the furnace, check the valve, the safety sensor and the ignitor.
Here is the guide to set up the Aquastat control:
Guide to Setting & Wiring Heating System Boiler Aquastat Controls

You are resetting it every time you take the panel off the front of the unit. It is locking out the circuit board. No. 1 reason for this is a dirty sensor. The flame sensor is the small rod-like part that senses the flame of the burner. It is usually pretty close to the hot surface ignitor (the part that glows bright when the furnace is igniting) Watch your furnace burner light. You will see the ignitor come on. The sensor is a little bigger than a pencil lead and it will be just a couple inches away from the ignitor.

Take the sensor loose and clean it only with fine sand paper or emery cloth. Chances are this will solve your problem. Hope this helps

Check your conection off the board to the initor and make sure the pins in the plug are set tight with the connection on the board. If you wire nutted the wiring together for the new ignitor you installed check these connection also. Make sure all wire strands at the wire nuts are in the wire nut and not touching or arching on the furances cabinate.

Check and make sure you have gas on to the unit and to the gas valve in the furnace.. Check for 24 volts at the gas valve at the furance. Check and make sure the knob at the gas valve is in the "on" position. If you have voltage at the valve than the valve has failed. If you have no voltaged at the valve than the circuit board has failed. One last thing you can check if you do have voltage at the valve. When the hot surface ignitor is glowing orange tap on the gas valve with the handle of a screw diver. Sometimes the valve sticks. Of coarse if this works it will probably stick again. Replace the valve in this case.

not sure what 21 is ,but if you are getting acode normally the code comments are on the door or the manufacture lable to tell you what they are...
i can tell you the operation and maybe that will help you out...
the unit should start off with the enducer draft motor small motor in furnace that pulls all the stale air out..then the negative pressure switch activates which should have a hose coming from the enducer draft moto to the pressure switch..once the pressure switch closes it sends power to the ignitor and the ignitor should glow once it gets hot a milivolt current is sent back to the gas valve and it opens up and fires...after it fires you should have a flame sensor small metal rod in the direct path of flame that keep sensing the flame and sends signal back to board.... unplug unit and watch it start where is it failing or in what part of the operation does it flash a code....

weatherking furance shuts off, i can hear a clicking noise from the gas relay, dont know if its the relay,pressureswitch or gas valve.

Just curious, there sometimes is a safety switch on the door to the fan unit so no one gets hurt opening it to change filters ...etc... make sure all the doors are on tightly

have the ac company check the actual blower wheel itself it may be slowly coming apart

On is just the blower fan nothing else, check to see if there is a reset button in the tstat or tripped over load inside the furnace, inside the furance is some safetys, you will see wires runing from each other and finaly to the gas valve, push each untill one of them clicks,then your furnace should start back working, but I would still have someone come out and check why it tripped, because as I said it is a safety, and when it tripps it is for a reason.