What's the usual causes for this code? I changed the vent valve, but don't want to start throwing parts at the car. On my last MC, it ended up being the fuel pump.

 

When I had the Monte that CEL code haunted me for years, had a smoke test done on it a few times but never found the leak, until one day I was offered a smoke test free at UTI as a training lesson and when you have 30 eyes looking they found the canister purge solenoid to be the only suspect, once it was replaced I never seen that code again

 

PO300
PO403
PO405
PO446 ✴︎ ⬅︎ ⬅︎ ⬅︎

All these Codes were triggered by a faulty FUEL TANK PRESSURE SENSOR!

Wait for the Low Fuel Warning before attempting to DIY.
Start engine.
Pull FUEL PUMP Relay ... Let Engine stall to relieve pressure in system.
Remove GROUND Wire from Battery.
With car Raised and SAFETY STANDS securely in place.

Support Tank with Jack.

(I made a "Jig" to attach to the Jack. Sawed the square bottom off from Milk Crate. Drilled holes through Hockey Puck. Mounted Puck to center of Milk Crate with Heavy Duty Zip Ties. Affixed carpet to other side of Milk Crate bottom for padding.)

Lower tank enough to access Sensor atop Fuel Pump.
Unfasten Pigtail from Sensor.
Unclip and Remove Sensor ••• Straight-up.
DO NOT BREAK THE END OF SENSOR OFF INSIDE PUMP!!!

If you need a Charcoal Canister do it when you replace Sensor.

Removing Relay to releive Pressure throws a Code which may require a MIL Light reset.