I fixed the Chevy Engine Running SEL Indicator aka Check Engine Light 3 times.
My 96 Blazer 4.2L had a P0440 once and the P0446 two times.
1st Time - P0440 Had dropped tank to replace the fuel pump and broke the plastic vent fitting on top of the tank. Easy to find, remove gas cap and blow air into the tank, you could hear the air rushing out..
2nd Time - P0446 Found a wire pinched at the Vent Solenoid (the one above the gas tank / spare tire area) that one was a little harder to find.
3rd Time - Replaced intake manifold gaskets to keep the antifreeze from dumping out. Then, 4 months later, the P0446 was back. Checked everything 3 times, and found the intake manifold bolts, all 8 of them had loosened to 88 inch pounds (they should be 133 inch pounds) re-torqued them and so far its looking good.
An intake manifold leak could cause the gas cap code.
EVAP Code 446 EVAP "the vacuum in the fuel tank when testing is taking more than 4 seconds for the vacuum to drop to the desired reading"... does this mean that it does eventually get there?
Check: Pressure Sender - Ground wire, a restriction in the venting /canister, or a defective in tank pressure sensor.