My Mansfield toilet model 160 runs periodically for no reason. What is the problem? Do you have step-by-step instructions how to fix it?

Not sure about your brand but many toilets have a flapper valve inside the tank which needs adjusting or replacing. Try this U tube link
How To Repair Mansfield Toilet

remove the lid and behind handle there will be a plastic nut tighten by hand

If this is the standard manfield 160 toilet I recommend replacing the fill valve with a Fluidmaster 400a. They are easy to install and adjust , and work very well.I have installed hundreds of these and have had zero problems. Hope this helps.

You prbably just need a new flapper valve. Buy one at Home Depot for a few bucks and it will, simply, snap into place at the base of the tank. Also, make sure that the chain is not too tight and cut off any excess chain after the flapper is installed.

If the toilet has been around for few just replace everything in the tank, they sell kits for this and would be worth your investment. It is not hard to do and you will have peace of mind knowing everything is new and will a good long time. If that is not your speed just replace each piece until you fix problem, starting with the bulb maybe its full of water.

Are your wast water pipes in your house vented through the roof? If they are maybe the vent pipe is clogged. At my father in-laws house the pipes are not vented through the roof like their supposed to be to vent sewer gas. they are not vented at all. Every time you flush it sounds like someone is downs stairs screaming. Something to do with air/ suction in the pipes.

mansfeild toilets are very common Korky or Fluidmaster makes a replacement ballcock and will change out easy I wouldnt fool around trying to find parts and replace the valve they run about 15 -$20.00 and replace the flapper at the same time

If this is the model am thinking it is, you need to replace the flapper. it goes down in open to far and gets bound.Or if you have hard water or calciaum buildup,it is making for a tighter opening. I hope this helps. :)

buy one online it will come with instructions i bet to.