Oven wiil not start

time to replace me thinks 21 years is a long run for anything these days

It may be on auto cook? Try unplugging the unit for 10 or 15 minutes then plug back in and see if it resets itself. Set the clock to correct time and start again. If it is still playing up, the touch pad membrane may be faulty. Don't use spray cleaners on these touch pads, it ruins them.

it has a fuse located behind the main control panel. you will see it after you remove the cover

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes.

The door switches are more than likely the problem, one or two will need to be replaced, I recommend a professional.

try resetting the clock. If it is digital, push the clock button and then the number buttons untill you have the proper time displayed...then push the clock button again. If your clock and timer are manual, try pushing in the clock lever and turning it 1/4 turn. If this works it would indicate that your timer has a start and finish time and that the oven is not working because the clock has rotated around to the finish time.

Ya those are a classic, really made tasty baking with convection heat but think it is old machine. Could be just a bad chord as unit runs about 12 amps. Like space heater, if they loose some connection start to make chord not good. I would look for another. I've seen a China one for around $100, and built-in ovens at Habitate for Humanity retail store with that feature for reasonable. Good to fix but when dealing with high amp points, they will wear out as they do with space heaters often.

Sounds like door switch problems or bad wires or connections at the door switches

Operating functions:
To set the clock, press and hold the clock/timer button for three seconds. A clock icon will appear on the left side of the screen. Input the time using the numerical buttons and then click the clock/timer button to save the time.
To set the timer, press and release the clock/timer button so that the display shows all zeroes. Enter the desired time using the keypad. Press start/stop to start the countdown. The countdown may be paused by pressing the stop/start button and resumed by pressing the same button again.
When countdown is complete, timer will beep continuously for thirty seconds and display will flash. Timer will then beep every ten seconds for ten minutes until the stop/start button is pressed.
This timer will also time up to time activities. To start count up, press clear/memory so timer reads all zeroes. Press stop/start and a plus sign will appear on the right side of the screen as the timer begins counting up.

cavity therm is bad probably, some people use the micro as a timer thinking they are using the kitchen timer and it burns out the cavity therm. get a qualified svc company to replace the therm, if it is the therm it wont heat or start or anything, if its just the timer thats not working then i suggest a new clock board.