Step #1. Add 2 ounces of DISHWASHER detergent ( NOT the liquid for hand washing in the sink ) to the tub even if you are using a gel or liquid form of this detergent). You don't need much. It adds ions and helps the plumbing rinse sparkling clean.
Any brand of dishwasher detergent should work. If you have the two chamber packet kind you need to borrow some regular stuff from a neighbor. You paid a lot for that special packaging and extra chemicals so you shouldn't waste it on these shocks.
Note: If possible use a powdered dishwasher detergent. Use the cheapest brand you can buy. You won’t pay for fancy rinse aids, enzymes and other stuff you don’t need that might hinder the cleaning.
Step #2. Add 4 ounces of regular salt on top of the Dishwasher detergent. This adds extra ions to help lift off the scum.
Step #3. Place a large load of rags or old towels in the drum. A really large load will increase the amount of water used resulting in a higher water line in the drum and it will also increase the wash time which will help a lot. Don't worry, the vinegar smell will rinse out before the cycle is over.
Step #4. Pour the one to two gallons of cheap white vinegar onto the old towels / rags in drum so they are well soaked (read note 1)
Note 1: If you pour the vinegar directly into the drum of the machine, it will be pumped out at the beginning of the wash cycle. The machine thinks it is water left over from the last load so it is a good thing for it to pump it out. So, you have to trick the machine by soaking the rags/towels with the vinegar.
Note 2: Two gallons of vinegar seems like a lot. It really works though. Use the cheapest vinegar you can find. To clean effectively, you need to offset the alkali nature of the detergent residue in the machine. Because vinegar is weak acetic acid you need lot of it. More than two gallons would be a waste though. Also, for any follow-up shock treatments (every three or four months) you should only need one gallon or less. Compare the total cost of shock tablets or other products to the fact that this method actually works....
Step #5. Run load with the longest and hottest water setting.
Note: If you minimize your detergent use you may never have to shock again. Well not ever, but some customers report year long periods where the shock was not run and everything was fine