Is an LPG Conversion Legal in the US?
Converting a gasoline car to run on LPG (propane autogas) is legal in the US only with a certified conversion system. The EPA recognizes conversions that use approved kits and keep the vehicle clean; an uncertified conversion is treated as tampering with the emissions system.
Why?
Switching fuels changes what comes out of the tailpipe, so alternative-fuel conversions fall under the EPA's tampering rules. The compliant path is a conversion system that has been demonstrated to meet EPA requirements for the specific engine family, which keeps the car's emissions within its certified limits.
California and the states that follow CARB add their own layer: an LPG conversion there generally needs a CARB Executive Order (EO) to be legal on the street, and a smog check will reject a non-approved system.
So an LPG conversion done with a certified kit and installed properly can be road-legal; a homebrew or uncertified conversion is not, because it defeats the assurance that the car still meets its emissions standard. Keep the certification paperwork with the vehicle.
What decides if it's legal
- An alternative-fuel conversion must use an EPA-recognized certified conversion system.
- In California and CARB states it must additionally carry a CARB EO.
- A non-certified conversion is treated as emissions tampering.
- The system must be installed correctly and keep emissions controls working.
Does it depend on your state?
The federal requirement for a certified conversion applies everywhere; California and CARB states add the Executive Order requirement, so a conversion legal in a non-CARB state may still need CARB approval there.
How other countries treat this
The same modification, judged by three different legal systems.
Related legality guides
Sources
- Vehicle and engine alternative fuel conversions (US EPA)
- Aftermarket, performance and add-on parts (California Air Resources Board)
Sources last reviewed July 2026.
This page is general guidance, not legal advice, on the rules in the US for LPG conversion. The detail varies by exact vehicle and changes over time - confirm with your insurer and the EPA, NHTSA and your state DMV before modifying.