How fix a toilet that keeps running?

Remove the tank lid and using the following list, examine each part inside the toilet tank.

• The Handle and The Flush Rod: The handle is what you push that commences the flush. Attached to that on in the inside of the tank is a horizontal rod called the flush rod that has a chain connected to it. This will sometimes come apart; a toilet repair, handle tighten or replacement. 
• The Lift Chain: Expanding from the handle horizontally, the flush rod will lift the attached lift chain. This will open up an outlet on the tank’s bottom, allowing the toilet to flush. The chain will sometimes work its way off the handle needing a minor toilet repair by reconnecting it.
• The Flapper: When you push the handle, the flush rod lifts the lift chain, which raises a rubber flapper, and that is where the toilet is allowed to flush. The rubber flapper acts as a stopper does in a sink. 
• The Flush Valve: Inside the tank bottom is a plastic part where that raised rubber flapper is allowing the toilet to flush, it is where the water leaves the tank and feels the toilet bowl. 
• The Overflow Tube: This part and the flush valve assembly are attached to this, the overflow tube, as a safety measure that prevents water from overflowing from the tank if the water supply valve doesn’t shut off. This is where the “running” toilet is usually repaired. 
• The Shutoff Valve: On the exterior of the toilet is a pipe for the water supply to the tank. Just above floor level is a shutoff valve. This is your go-to when water is running out of the toilet and you can’t stop it. 
• The Supply Tube: This is where the water runs through between the shutoff valve and the tailpiece of the water supply from the tank’s bottom. 
• The Water Supply Valve: Inside the tank is where the water supply valve is located, a vertical assembly that is mounted on tank’s left side and is the center of the toilet’s operation. Fresh water supply comes from this to refill the tank after each flush.  
• Float Cup: There is a float device inside any supply valve that senses the water level, and with a toilet, it is inside the tank. When the fresh water has filled the tank to the proper level, the float cup shuts the water off. When a toilet is constantly running, a simple toilet repair is to replace this part. 
• The Refill Tube: In the center of the tank, there is a small tube that clips to the overflow tube. This flexible tube is the refill tube and it allows a trickle of water to the toilet bowl as the tank refills.
• The Toilet Bowl: It is here where all the human action takes place, the throne! This is bolted to the floor and the curved trap on the back side works the same as the P trap under the sink, preventing sewer gas to fill your house. structure that works just like a sink drain trap. 
• The Wax Ring: Under the toilet bowl, before it is bolted down, the wax ring is placed to seal u where the toilet bottom and the floor meet. It is watertight, keeps the toilet water from escaping onto the floor. When you have water leaking from here, it is a major toilet repair that needs professional attention that offers toilet repair and installation. 

Can you replace the inside of a toilet?

Anytime a toilet is constantly running, the rubber flapper is usually worn out. There are two gallons of water in a toilet tank 24/7/365, so it isn’t surprising these metal, plastic, and rubber parts wear out.  When they do, they can wreak havoc on a toilet and the water bill. 

This doesn’t mean you need a new toilet though. You can use the above descriptions and follow online videos for “How do you fix a toilet tank?”  and determine which part of parts need replacing, or you can get a kit that will include all the internal tank parts and replace all of the parts. You can do a toilet repair just replacing those specific parts, and it is possible that you need a toilet repair without a flapper.  There are instructions online how to do this as well.