Sewer clean outs are really handy whenever you have a sewer main clog or when you have a tough stoppage to clear. One thing to keep in mind is that clean out caps can leak just like any other plumbing fitting. It's a good idea to know where all of the clean outs are located in your home so you can check them periodically.
Why I Check for Leaking Clean Outs
While crawling under a house recently I came across three clean outs, and all three caps were leaking. Even though these leaks were minor, this isn't something you want to ignore (this is a sewer line, after all). Since the clean outs are usually located in the basement or crawl space under the house, it isn't a problem you are likely to notice. One way to catch the problem early is to check the clean outs as part of your routine plumbing maintenance.
Why Clean Outs Leak
Clean out plugs are threaded—for one very good reason: they must be removable to provide access to the pipe. In fact, that's the sole purpose of a clean out fitting. But the threads are also the downfall of the clean out, in terms of holding liquid, that is. Threaded fittings have to fit really well to be liquid-tight, and when it comes to cheap plastic clean out plugs, the tolerances simply aren't close enough to do the job in many cases. Old brass clean out plugs can leak, too, for the same reason.
Beware Leaky Clean Outs
While it's a good idea to check for leaky clean outs, that doesn't mean you should always remove a clean out that seems to be leaking. Why? Because if something is leaking out of the clean out, there's a good chance the drain pipe is full of wastewater (and you know what's in wastewater). Drains are designed to get rid of waste without pressure; they're not designed for holding lots of water and the pressure it can impose. (And that's one reason why the threads on clean out plugs don't have to be as tight as those on water supply piping.)
If you remove a clean out that's leaking, and the drain happens to be backed up, you could unleash a terrible, swift, and stinky stream of wastewater into your home. Best to play it safe. If all the drains, including the toilet, are working normally and not at all slow, there probably is no backup, and it's ok to remove a clean out. Just do it slowly, and check for extra liquid coming out as the plug unthreads. Also, stand to the side before you make the last few turns on the clean out plug, just in case there's a gusher.
How to Seal a Leaky Clean Out
Clean out plugs usually can be sealed with plumber's tape, or pipe thread tape, (commonly known as "Teflon tape," although there's no such product of that name) or with pipe dope, or pipe joint compound. Simply wrap the threads of the clean out plug a few times with the tape, or wipe on a liberal coating of pipe dope, and reinstall the plug. Remember to wrap the tape clockwise (when viewed from the underside of the cap) so the tape doesn't bunch up when you thread on the plug.