The upstairs toilet has been backing up and emptying into the upstairs bathtub.
The only other toilet in this house is in the basement which is working. The house is connected to a town sewer system. What could be the cause?

Sounds like there is a clog downstream of the toilet.

I would have you RENT a snake and then send the electric snake down the drain to see if could retrieve whatever it is that is clogging the line.

You could snake from the roof vent if roof relatively flat... OR could lift the toilet and snake the drain from there, too.

If upstairs toilet is on the first floor, then may find a cleanout in the basement UNDER the bathroom that you can snake from.

Overall... definitely need to snake the drain... I would start by using a retriever head and see if I could pull anything back.

When I pull a toilet to snake the drain I fill the drain line to almost overflowing so that when I hit the blockage I can tell by watching the water...

Let me know if need more info... glad to help here.

Mark

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You need to get blockage clear but meanwhile you can install non return valve on drain pipe of bath it will stop toilet water entering bath pipe but it will drain water from bath tub.

Fjaved this just is not correct advice to give in this situation. There is blockage in the drain after the joint between the tub and toilet and it needs to be rodded, no such valve is used in household plumbing.

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Originally Posted by diland

The upstairs toilet has been backing up and emptying into the upstairs bathtub.
The only other toilet in this house is in the basement which is working. The house is connected to a town sewer system. What could be the cause?

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Hey, Best guess would be to check all of your traps make sure they are not clogged up, 9just in case you don't know what traps are, they are the part of the piping that look like this, look at this link. Google Image Result for http://www.dekalbsanitarydistrict.com/dsdodor.jpg I know it a lot, just try to copy and paste but not all traps have a cleanout like this one, if not just undo it and clean it out.

Good research by Jungle,
However, since "P" traps are self scouring the chances are the blockage is downstream from the tub. The solution would be to snake the bathroom branch, 1) from the lavatory roof vent **or** 2) pull the toilet and snake from there. If you snake from the roof vent put out enough cable to reach the base and 15 feet more. Good luck, Tom