The P0463 is pointing to a fault in the Fuel Level Sensor. This sensor is what sends a signal to the instrument panel showing the fuel level. It's actually a fairly common problem. This sensor is mounted to the side of the fuel pump assembly, located inside the fuel tank.
The P0496 is pointing to a fault in the Fuel Tank Pressure Sensor. This sensor is located on the top of the fuel pump assembly.
Check the wiring harness to the fuel pump assembly for any broken or loose wires. If the visual inspection is good, suspect a faulty Fuel Level Sensor, and the Pressure Sensor fault may just be a by-product of the faulty level sensor. If the P0496 returns after the level sensor replacement, the pressure sensor may need to be replaced as well.
is there a way to test the fuel level sensor
Unfortunately, GM does not provide a "Testing" procedure, however, with an Ohms meter, the Ohms value should move, smoothly, when the level arm is raised and lowered. When you get the p0463, its usually a straight forward code that the sensor is faulty. Like I say, it's a very common problem, as the "teeth" on the level sensor that meet the "striker" tend to wear out. I would definitely double-check the wire harnesses going to the fuel pump assembly before condemning the sensors, due to the fact that they are both powered by the same harnesses and to have two faulty sensors at the same time very well could be electrical. Again- if the visual inspection is good, i'd assume the Level Sensor could be setting the pressure sensor code.