Folks,
After three years of intermittent and perplexing behavior from the Jenn-air down-draft range that came with my house, I finally broke down and installed a new relay board and oven temperature sensor. Not only are all the problems gone, I now actually see how the range is supposed to work.
- Oven now holds temperature and is accurate +/-5%.
- Previously, display would indicate set temp, but actual temp would be far lower.
- Oven fan runs at lower speed and is almost silent
- Oven fan shuts off immediately when oven cools down; before it would run for hours.
- Alert bell now works
I’m a retired electrical/electronics engineer and undertake such projects fearlessly. It’s difficult to judge whether an ordinary "handy" person should undertake this project. Some of the wiring to the relay board is via multi-wire J-connectors that cannot be installed incorrectly–so these are a snap. However, the power-controlling wires attach with old-fashioned spade connectors, which sometimes require an extra-ordinary amount of force to remove and re-install. I’m accustomed to working with such connectors, but do-it-yourselfers might be disconcerted by the hefty force required to separate and reunite the male and female halves.
Before beginning, I recommend that you take several photos of the board AND make a crude drawing that shows where each wire plugs into the board.
Because my oven was not holding the displayed temperature, I also replaced the temperature sensor in the oven. It’s a 10-minute task and the new part seems better-made than the original.
So in one swoop I’ve eliminated two negatives in my life: a stove that doesn’t work well and a wife who nags me incessantly to fix it. 
Good luck and thanks for the folks at Appliance Parts Pros.
Joe C.