*Disclaimer: State regulations can sometimes go above and beyond Federal/EPA regulations. We will try to provide the best information and links to your specific state. Compressed Natural Gas can be dangerous, so don't try anything unless you know what these terms mean.
The CNG world has a few different standards and certifications you should know about. It can get confusing, but SkyCNG will try to keep it simple and relating to what you're trying to look for.
Here is a complete list of Federal and State CNG conversion incentives.
What is an EPA certified kit? Does the EPA certify cylinders? How do they know I'm even getting a CNG conversion?
These are all good questions and you can find more info in the forum/FAQ. The EPA does indeed "certify" CNG conversion kits specific to certain year model vehicles and engines. There are many makes and models that are "EPA Certified". Your local qualified CNG conversion shop will know if your vehicle can be converted. The EPA Certification process is very expensive and can cost hundreds of thousands of dollars. A few thousand dollars of your conversion fees will help cover the cost of this root canal.
Ford, Honda, and other big named manufacturers have to go through this for the bi-fuel and CNG dedicated vehicles. When you buy a car or car conversion kit that is EPA certified you would then qualify for all applicable federal and state tax rebates and kickbacks. An EPA Certified kit would include an NGV2 (and most likely FMVSS 304) CNG cylinder. Be aware of this before you get a car conversion, because you may not be eligible for any state or tax rebates if you get an "unapproved" CNG conversion kit. The kit may work just fine, but you won't qualify for any rebates.
UPDATE: August 2012 The EPA released a new set of rules for their certification. It is a little more clear to understand and they are making an attempt to loosen up when it comes to CNG car conversion. They created three program elements with the categories: New, Intermediate Age, and Outside Useful Life. "New" vehicles are defined as the current calendar year minus 1. These "New" vehicles can be issued an EPA Certification letter/number if required where you live. The other two categories do not require any type of formal certifications. This means if you have a vehicle 2 years old or more you do not have to fall for the "EPA Certified" CNG car conversion threat. SkyCNG conversion kits will be legal and legit on your vehicle. Contact SkyCNG with any questions about your CNG car conversion. For futher details and info you can visit the EPA website and look for EPA420-F-11-006, March 2011. Sorry California, but CARB still requires their own application and certification even if your vehicle is over 2 years old.
There is no such thing as an "EPA Certified" conversion shop or installer. Also, there is no such thing as an "EPA Certified" CNG cylinder. Further, two different cars could have the exact same CNG conversion kit but only the vehicle and engine that qualifies as "EPA Certified" will receive the designation as such. See the EPA short list of eligible vehicles and conversion kits. Automobile manufacturers and aftermarket converters pay several hundred thousand dollars for the EPA to test and "approve" each model vehicle AND engine. Conversion could be much more affordable if not for these fees. If your vehicle is not on the list you can still buy a kit and get converted. You will likely not be eligible for any tax incentives or rebates by doing this... depending on what sort of laws get passed on the state or federal level.SkyCNG does not encourage people to get non-EPA Certified conversion kits if they are not legal in your area. You could get fined.
The State of California has decided that the EPA Certification was not enough red tape for the CNG industry. All vehicles registered in California MUST use a compressed natural gas conversion kit that is approved by the State Air Resource Board (CARB). This will mean higher prices for CNG conversion. Read about it through this link to that State CNG info website.
ANSI NGV2-2007 (the CNG cylinder standard that matters to most of us)
This is the American National Standards Institute code and specification for CNG vehicle cylinders.
The ANSI NGV2 has been around several years and most recently amended in 2007... albeit the changes were not much different compared to the year 2000 standard. You can buy the PDF of this CNG standard here.
What the consumer should know is that this NGV2-2007 certification is critical in finding a reliable vehicle cylinder for your car conversion. An ASE certified mechanic and experienced CNG vehicle conversion shop would not convert your vehicle unless the cylinder meets the NGV2 standard. There will be a label on the cylinder that states the certification as well as other critical information. Be sure it says NGV2-2007. A manufacturer has to pay for extravagant testing for NGV2-2007 compliance.
FMVSS 304 This applies to cylinders for new OEM vehicles and usually will not apply to typical aftermarket CNG conversion kits. To get this qualification it must be crash tested in addition to NGV2 specification. The Federal Motor Vehicle Safety Standards department makes sure that your vehicle was built to safe and appropriate standards. Here is a PDF of FMVSS 304. If Honda wants to start using "Company ABC" cylinders in their new CNG cars, then "Company ABC" would only be able to sell them cylinders that are FMVSS 304 certified. This certification is a little over the top and goes above and beyond NGV2-2007 in a few small ways, but the big difference is that a manufacturer has paid a ton of money to get additional testing to they can use the FMVSS 304 certification and blessing from USDOT. It is sort of redundant to the NGV2 standard, but oh well. Write your congressman!
ISO 11439 The International Standard required for gas and high-pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles. ISO is probably a little more comprehensive in its specifications that other standards. You could drive around with an ISO 11439 compliant cylinder and feel safe, however you will not be "approved" for use on the USDOT roads and highways unless your cylinder sticker at least says "NGV2 Certified" on the label.
NFPA-52 This is the "fire code" (National Fire Protection Association) department of ANSI regarding NGV and alternative fuel vehicles. Your CNG car conversion kit must meet the specs of this standard...although nobody will ever know unless you get into a wreck and become liable for something. If you are using an experienced ASE mechanic who knows his way around a CNG conversion kit, then they will know how to meet the NFPA-52 code. This code pertains to not just the cylinder installation, but the complete proper installation of your vehicle conversion kit. You can see the actual document here.
*Note: The EPA operates unilaterally and any conclusions of SkyCNG today are subject to being completely wrong tomorrow.