Rest the propane tank in a propane tank caddy or "nanny" in the trunk of your car. Available through propane tank suppliers such as U-Haul, the "nanny" secures the tank in an upright position and prevents it from falling or rolling during travel.
Close the cylinder valve and seal with a plastic plug. Even if the cylinder appears to be empty, residual propane settled at the bottom of the tank will expand with temperatures and may escape from the tank. A plastic plug, available through propane suppliers, protects cylinder threads and prevents propane from leaking into the car interior.
Transport the propane tank to a refill station. Many gas stations and garden centers have filling stations where a trained attendant will refill the propane tank. Never attempt to fill or modify the propane tank yourself.
Ask the attendant to inspect the propane tank. If it's damaged, rusted or doesn't have an overfill protection device (OPD), exchange it with the refilling station. Most stations have replacement tanks that are inspected and up to code. According to the National Fire Protection Association, an OPD shuts off the filling device when the tank reaches 80 percent capacity. As of April 1, 2002, an OPD is required on all propane tanks between 4 and 40 pounds capacity, which helps to prevent propane leaks and potential explosions.
Check the tank for any leaks if you smell propane. While the leak can simply be nothing more than residual from the valve, as additional security, you should check before you hook it up. A soapy "bubble test" is a simple yet effective test for leaks. Before the test, close the valve and insert the plastic plug. Make a heavy, soapy solution with dishwashing soap and pour enough over the regulator, valve and threads to coat. A leak will produce bubbles from the aperture. If you find a leak, call the supplier immediately. Do not transport the tank until you receive instructions from the supplier.