Household sewer lines get blockages from tree roots invading the line through coupling joints, excessive use of toilet tissue, cooking oil sticking to the pipe walls and many other things. When a blockage is stubborn and won't shift, rodding the line will usually solve the issue. Power-driven sewer snakes have a flexible rod, that once inserted into the sewer, is turned to free the blockage. These tools are generally rented from do-it-yourself rental facilities.
Place an adjustable wrench around the centre nut of the end cap on the sewer line's Y access fitting. Turn the wrench counter-clockwise to remove the cap. Pull roughly four feet of flexible rod from the sewer snake's casing and push it down into the access hole so that the rod enters into the sewer line. Keep feeding the rod into the sewer line until it meets the resistance of the blockage.
Plug the sewer snake's cable into a GFCI (Ground Fault Circuit Interrupter) outlet - all bathrooms should have GFCI outlets installed, as required by building code regulations.
Switch the sewer snake's dial to the counterclockwise setting. Press the start button to rotate the flexible rod inside the sewer line. Pull the cable out of the sewer line a few inches, and switch to the clockwise setting.
Push the flexible rod down further into the sewer line a few feet, then turn the sewer snake's setting back to counterclockwise. Pull the rod out a few inches as before, turn the setting to clockwise and push the rod in a few more feet. Continue in like fashion until the rod is fully in the sewer pipe - for longer 50/100ft. rods, make sure the rod is at least six feet past the blockage.
Switch the snake's setting to counterclockwise and slowly pull the rod out of the sewer pipe. Turn off the power the sewer snake, clean the rod with a rag and feed it back into the casing.
Wrap white sealing tape twice counterclockwise around the end cap's side threads, and screw the cap into the end of the Y access fitting. Tighten it in place with the wrench. Run water down into the sewer line to remove all remnants of the blockage.
If working on a sink blockage, unscrew the nuts on each side of the p-trap's bend by hand, and pull off the bend (the P-trap is located under the sink). Feed the end of the rod into the sewer pipe coming out of the wall that the trap was attached to, and follow the same instructions as above. Afterwards, replace the P-trap by repositioning the bend in place and tightening the nuts by hand.