After-market conversion kits, for converting standard engines to operate on E85, are generally not legal in U.S. states subject to emissions controls unless you get your converted vehicle independently EPA certified. This is despite the fact that the exhaust emissions from any such converted cars are improved by utilizing higher percentages of ethanol in the gasoline blend. Unfortunately, EPA certification costs in excess of $23,000 and you additionally have to prove that your vehicle will maintain low emissions for at least 50,000 miles after the conversion. Most individuals won't give up their vehicles for the requisite 50,000 mile test period. Likewise, conversion kit manufacturers generally don't certify their kits due to the onerous and expensive burdon of these laws. The kits would have to be tested with every model vehicle for which they are to be sold. If a kit is already certified as described, the EPA Federal Test Procedure for an individual's conversion costs $750.00. One can request a reduction of payment of down to 1% of the car's added retail value due to the conversion. A minimum fee may apply if the value added is not seen to be very high.

Similarly, U.S. Federal law prohibits the manufacture of many such conversion kits for sale in the U.S. unless they are EPA certified. The origin of this ban dates to when conversion kits for using compressed natural gas were originally sold. The ban was enacted to prevent the sale of such conversion kits due to safety concerns. This ban on the manufacture of kits is at odds with the fact that these kits, once existing, are legal in all states but CA, and most states offer some sort of tax break for converting your vehicle (See tax breaks.)

One Brazilian after-market kit is available legally in US States that do not have restrictive emission controls. The kit will permit the conversion of 4, 6, or 8 cylinder engines to operate from fuels ranging from pure gasoline to a mix of gasoline and ethanol to pure ethanol, including E85. It operates by modifying the fuel-injection pulses sent to the fuel injectors when in 'A', or alcohol mode instead of 'G', or gasoline mode. (In 'G' mode, no modification to the fuel-injection pulses is performed.) This conversion kit modification serves to extend the control range over which the ECU can adjust the air-fuel ratio to achieve an oxygen sensor reading measured before the catalytic converter that falls within nominal stoichometric ideal combustion limits. The general belief is that this conversion kit operates in its 'A' mode simply through lengthening the individual pulse-widths of fuel-injection pulses, thereby increasing fuel flow per injection pulse by roughly 30%, whereas in 'G' mode, it acts simply as a straight pass through for fuel-injection pulses. Due to the nature of this kit, it is fully reversible (see below for other approaches).