The drier on our Whirlpool 1200 awz 412 will not start, even after several rounds of switching off & on & of opening & shutting the door.

Might want to check for an obstruction in the outlet of the exhaust

Well if you had mention that in the first place I wouldn't be asking you. No wonder your trying to fix the problem by yourself because with that attitude you can't get a technician to come out for service.

Electric dryer no start does nothing

Check dryer Terminal block prongs both outside prongs should give combined 220, and 110 each if u check 1 outside & 1 center (ground) prong. Also check house electrical outlet for full voltage.

CHECK THERMAL SHUT OFF THERMOSTAT USUALLY LOCATED ON THE BLOWER. for OHMS or resistance with a meter. Should show a closed circuit. Ohms WHEN THIS FUSE BLOWS IT WILL SHUT DOWN EVERYTHING.

NEXT Test the PUSH start switch with meter for OHMS. Should show OHMS when pushed in or on start, and no ohms or infinity when in off position.

If you hear a humming/clicking sound from motor area you more than likely have a bad motor.
Try this test out to see if u have weak windings or relay/capacitor on motor.
Open the dryer door and turn by hand the drum while machine is on ON position. And you or someone else presses the door engagement switch in. If it starts up then u have bad windings

Another thing to check is the belt safety switch. With a multi meter.
Especially if u just replaced the belt. The belt safety switch is attached to the Tensioner and motor. IT LOOKS LIKE A DOOR SWITCH SOMEWHAT a little black pc of plastic with 2 prongs going into it. Sits at bottom of motor. Under the Tensioner. Attach meter clips to the 2 prongs on switch, Then when the little white push button is pressed in it should show continuity and none when released.

Lastly check ur DOOR SWITCH for continuity with a meter when the button is pushed in the (on/Door closed position,) it may have gone bad.

https://www.youtube.com/watch?v=sTRP6kHqAFs
Hey GOD IS GOOD !!!! WHICH IS WHY MY ADVICE IS FREE

With a washer and drier thay do , a separate drier would be better for you

I also need to replace the door seal on my Whirlpool AWZ 410 washer. Does anybody have detailed instructions of how to do this for this particular machine model (not generic instructions) & know if it is necessary to remove the front panel on this machine in order to remove the existing seal please?

You will need to replace the motor control board. It is malfunctioning on the high speed control.

When start button is pushed there is a buzzing noise but dryer will not start.

Two things can cause this one is the vent hose and or the vent going out side. If it (the hose or the vent) is stopped up this will happen. Also The Drier has a protection switch mounted in the burner area. This switch is going bad. It is there to shut down the drier in case of a problem it is kicking in/out too soon most likely it is going bad. When it shuts off, open the door let things cool off for a minute see if it will restart.

http://shared.whirlpoolcorp.com/assets/pdfs/literature/Use%20and%20Care%20Guide%20-%208522243.pdf

if possible, with the door open, push the limit switch the door normally hits to let the drier know the door is shut. That switch may be bad.

It could be as simple as a door switch adjustment. Get it fixed under warranty.

FIRST switch off and unplug.

Pull machine out and remove lid by undoing self tapping screws at the top back. May have to slide lid back a bit to lift off.

Internal catch mechanism will be reachable from top for manual release. You may need someone to push door inwards while you release catch.

Good luck