Fuel Injector Seal/O-Ring Kit - O''Reilly Auto Parts4. 5. 6. Last. A fuel injector seal or fuel injector O-rings seal the injector to the fuel rail and intake. The O-rings operate under high pressure, and can dry out and fail over time. When this happens, fuel can leak from the injector or fuel rail. You may notice a fuel smell in your engine bay, or notice a fuel leak.