The CNG cylinders are usually the most expensive parts of a natural gas conversion kit. These parts carry labels which specifically show they meet the standard set forth by the Department of Transportation. For reference, the standard is FMVSS 304 which will be visible on the label with a DOT symbol stamped on the label also. All labels have an expiration date. This date is considered when the cylinder can no longer be used and must be removed from the vehicle. If you try to pass emissions with an expired cylinder you will fail emissions based on the date, not on anything else.
It is the law the CNG cylinders should be inspected every 36,000 miles or every three years, whichever comes first. You are required to carry documentation with the vehicle for proof of passing an inspection with the natural gas conversion kit. There can be a big fine attached if you no longer have the paperwork. It is best to keep this information right along with your proof of insurance and registration.
A conversion kit for natural gas is the best solution to saving money and meeting the solutions of the rising gas prices. Federal standards are set forth which must be met for purchasing, the actual kit, and more. The DOT will put a label on the DNG cylinders which will show proof of inspection and include an expiration date. These are safety standards you must abide by when you are running on natural gas. Always be sure to buy a certified kit that will properly pass emissions and have an inspection on the kit every three years.