In recent years, the State of California has become the unofficial capital of plug-in hybrid technology. But proposed certification standards from the California Air Resource Board (CARB) could create an obstacle for small companies selling plug-in hybrid conversion kits. The kits—which allow owners of today’s hybrids to boost their fuel economy to as much as 100 mpg or higher—could have unintended negative consequences, according to CARB. Tests of some plug-in hybrid conversions at the Argonne National Laboratory revealed increased levels of air pollution. Apparently, changing a hybrid’s battery control system can also alter the vehicle’s electronic emissions system.

CARB officials want to see additional testing and to require consumer warranties for the kits—regardless of the cost. The agency currently requires that all aftermarket parts affecting a vehicle’s emissions meet high standards, but no certification process has been established for plug-in hybrid conversion kits.

Consumer demand for plug-in hybrids is rapidly climbing. Major auto companies like GM, Ford, Daimler, and Toyota say their plug-ins are coming, but not for at least a couple of years. Plug-in conversions of today’s hybrids could fill the gap.

The proposed regulations (PDF) would require conversion companies to follow a test regimen similar in scale to those followed by automakers for new vehicles. They would also force manufacturers of plug-in conversion equipment to provide warranties of seven to 10 years.

The work of plug-in cheerleaders and kit manufacturers has been a key factor in inspiring—or prodding, depending on your view—major car companies to pursue plug-in hybrids. But rules designed for the big car companies to bring a vehicle to a mass market may end up preventing these small conversion companies from putting the first wave of plug-in hybrids on the road.