Pontiac Repair Questions? Ask a Mechanic for Answers ASAP
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The P0455 is "EVAP System Large Leak Detected" - this can be something as simple as a loose, missing or faulty Fuel Cap. Even putting fuel in the vehicle while it is running will set this code. If the fuel cap check ok, do a visual inspection looking for any disconnected or ripped vacuum lines.
The P0461 and P0463 are both pointing to a fault in the Fuel Level Sensor (your fuel gauge should not be working correctly). Check the electrical connector at the top of the fuel tank going to the fuel pump assembly for any loose, broken or rubbed through wires. If the visual inspection checks ok, suspect a faulty Fuel Level Sensor (actually quite a common problem with GM fuel systems.). This sensor is mounted to the side of the fuel pump, inside the fuel tank.