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The smell of gas is one of the best indications that you haven't successfully fixed a gas leak. While you might not always be able to smell a leak, you should pay attention to see if you do notice the smell of leaking gas.  Gas smells like Sulphur or “rotten eggs.” The smell of gas is made by mercaptan, an additive that is intended to help humans smell it. Shut off the master gas switch for your home and consider calling a professional if you smell gas. The gas main switch will be located next to your meter -- usually on the side of your home. Carbon monoxide detectors and gas detectors are normally used to alert people to gas leaks in their homes and businesses. While most detectors are stationary devices meant to detect an accumulation of carbon monoxide or other fumes, you can purchase portable detectors.  Position your stationary carbon monoxide detector near the repair you've made. If you have a portable detector, move it near (within several inches) to the repair. If there is a gas buildup near the detector, it will sound an alarm. This works because the detector solution or soapy water will bubble due to the force of the leaking gas. Ultimately, this is a simple and effective way to find out if you've successfully fixed a gas leak.  Commercially produced gas leak detector is specially formulated to stick to tight connections. If you want to create your own solution, try mixing a few drops of dish soap into a cup of water and spraying it onto the gas connection. If the solution bubbles, you likely failed to repair the leak. Gas detector may be available at your local home improvement store, specialty stores, or online.

Summary:
Notice if you smell gas. Use a carbon monoxide or gas detector. Spray liquid gas detector or soapy water on a pipe or appliance.