Because of this, we get many e-mails from people asking whether their BMW, Audi, VW, Nissan, Toyota, Ford or Chevy can be converted to CNG. The answer is both no and yes.
The EPA highly regulates conversions
The Environmental Protection Agency strictly regulates how your car's fuel system, and especially the exhaust system, works. Changing your car to operate on CNG (or both regular gas and CNG, which is more common in conversions) often alters how this system works. That is fine as long as the system has been rigorously (and expensively) tested on a per-engine basis by the EPA to make sure that key emission levels have not been increased and that the sensor system in the exhaust area is still working correctly.
There is an additional level of certification called CARB (California Air Resources Board) that is even more expensive and complicated than the EPA certification. In many states (including California) CARB certification is required above the EPA certification, even further limiting the number of certified conversions.
There are very few approved conversions
As a result of these strict test requirements and the expense of getting a CNG conversion kit certified, there are very few EPA and/or CARB approved CNG conversions, and the conversions are relatively expensive, costing from $8,000 to $16,000. NGV America has a document that lists all of the approved conversions. Some vehicles that are on that list include the Chevy Silverado, GMC Sierra, Ford Crown Victoria, Ford Grand Marquis, Ford Taurus, LIncoln Town Car, Ford F150, F250, E-350, E-450 and others. However, even for the ones listed here, the conversions are not available for all model years. Please check the document to see if the car you are interested in is included.
My car is not listed, can I still convert it?
Even if your car is not on the list, there are plenty of companies who will sell you a CNG conversion kit for just about any make and model car. Unfortunately, however, because of the steep costs of EPA certification, these kits are not EPA certified and certainly not CARB certified. As a result, installing these kits can become a legal issue in many states and could result in fines or other problems. With some kits, there are also safety concerns as you are dealing with a flammable gas at very high pressures (about the same pressure as a mile and a half deep into the ocean.)
So the answer to the question "Can I convert my car to run CNG if it is not on the list?" is probably you can, but it definitely comes with risk and liability.