1) Test disposal is getting electricity and switch is not bad.
Pull out switch so you can test wires:
Switch in ON position.
Use ordinary tester and test each brass screw to bare ground wire.
Tester should light up on each screw.
2) Next open wire connection at disposal and test & check wiring.
Be careful under the sink. Do not touch anything metal while working on live electricity.
Stay on completely dry surface.
3) In event of bad disposal, I wrote installation tips.
These tips do not include installing new disposal and taking old one back in same box as if it was a defective product.
When I worked apartment maintenance, we replaced disposal with same size and brand disposal.
That way we didn't have to change out sink basket or move pipes around under the sink.
How to: Disconnect 2 nuts on p-trap and trap falls loose.
Rotate steel connector disk that holds disposal to sink basket, and disposal comes off.
Before installing new disposal.
Attach electric wire to new disposal.
Attach dishwasher tube to new disposal. Knock out plug inside dishwasher opening.
Push new disposal up into place, and attach to old sink basket.
Use new connector washer.
Attachment ring catches at three points, all three have to connect.
You can use bottle jack to lift disposal if you have problem.
Re-assemble tubular pipe.
You might want to install new sink basket that comes with new disposal because it looks shiny and new.
Buy a tub of plumbers putty also.
Remove old sink basket and clean sink opening.
Put finger-size roll of plumbers putty around sink opening, and then push new strainer basket into plumbers putty.
Tighten basket from below, wipe away excess putty around sink opening.
Install new disposal as described above.
Tips for tubular pipes:
When taking apart tubular pipes, start at p-trap and take apart as few pieces as needed to do job.
On image above, loosen both upper and lower nuts, trap falls loose, and then work on disposal.
Any tubular drain pipe that gets moved while working under sink needs to be disassembled, cleaned and re-assembled.
Metal pipes have rubber washer >> always use new rubber washers.
Plastic pipes have plastic washers > plastic washers can be cleaned and re-used.
Plastic washers have flat side that goes toward nut, tapered side away from nut.
When reassembling tubular pipe, we use rectorseal on washers and joints.
Slap a good amount on washer just before pushing washer in place and attaching nut.
Rectorseal will ensure tubular drain pipes won't leak.
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Also take advantage of fixyas phone service.
For a price, fixya expert speaks with you over phone while you work on disposal.