Check Engine Codes P0441 & P0446

[thun]

I've got a 98 Prerunner 4-cyl.

After driving on some pretty rough terrain, I noticed my check engine light had come on. I went to a store to have the codes read and I got the following codes:

P0441
The PCM has determined that the purge flow in the evaporative emission control system is incorrect for the current engine operating conditions.

and

P0446
The PCM has determined that a malfunction exists in the electrical circuit for the vent control in the evaporative emmision control system.

I've read elsewhere that P0446 is caused by a loose gas cap, although I'm not sure that's the problem. As for the first one, I'm stumped. I've had it suggested than the O2 Sensor might need replacing, as it is somewhat fragile and can break from a good shock. Also, I've noticed a slight decline in my gas mileage (~5%) since this incident several months ago.

Any other ideas? Thanks in advance.