Undefined Fuel-reserved For Proprietary – Newest

The “reserved” portion is often protected—cannot be used by auxiliary heaters or power take-off (PTO) units. Some off-highway equipment (mining haul trucks, agricultural sprayers) have a that only unlocks with manufacturer software or a paid feature.

These are standardized across all makes and models (e.g., a P0300 misfire code). undefined fuel-reserved for proprietary

Note that "Undefined Fuel" or "Fuel Other" codes (like 076–098 or 099) may not automatically trigger tax-exempt processing in certain fleet programs. Note that "Undefined Fuel" or "Fuel Other" codes

The vehicle was filled with something like a specialized biodiesel blend, a racing fuel, or a new alternative fuel not yet broadly categorized. System Customization: It’s a simple, "low-tech" fix that often clears

In many cases, proprietary fuel codes are triggered by a loose or failing gas cap. It’s a simple, "low-tech" fix that often clears "proprietary" EVAP codes.

As alternative fuels like E85 (flex fuel) or high-blend biodiesels enter the market, older POS (Point of Sale) systems may not recognize the new national codes, defaulting them to a "reserved" or "proprietary" status.