Fuel Injector Misfire 0304 error 2006 Pontiac Grand Prix, 3.8L, base

Check the intake manifold bolts to make sure there snugged up. You on the right path for diagnosing the problem. Other causes, I didn't include what you have done already... Fuel injector 4 harness is open or shorted, Fuel injector 4 circuit poor electrical connection, Ignition coil 4 harness is open or shorted, Ignition coil 4 circuit poor electrical connection, Insufficient cylinder 4 compression, Incorrect fuel pressure, Intake air leak...Here is some info on your other code....P0455 PONTIAC - Evaporative Emission Control System Leak Detected Gross Leak...Possible causes; Missing or loose fuel cap, Incorrect fuel filler cap used, Fuel filler cap remains open or fails to close, Foreign matter caught in fuel filler cap, EVAP canister or fuel tank leaks, EVAP system hose leaking, Fuel tank leaking...The P0455 it’s a very common code, because the code is set if the gas cap is loose or missing. Start by tighten the gas cap and clearing the code. In most cases the code can be clear by disconnecting the car battery for about 30 minutes.

for your po 455 code. there is a rubber line, about 1.25" in diameter that runs from the evaporator canister to the fuel tank. about half way along the hose it is cut in two and reconnected with a plastic pipe on the inside, maybe for production reasons, rather than using a continuous hose. you will find this hose by going under the left rear wheel. on my 04 this connection was leaking air. use two jublie clamps to seal the tubes around the pipe. that solved my air leak. good luck