HELP! 99 camry with bad air/fuel ratio sensor- AGAIN!

[sabbates]

All-

I've got a 4 cylinder 99 Camry LE with 80K miles.

About 6 months ago, the check engine light came on with "too lean" message, I believe it's error code P0174.

The dealer said this was due to a bad air/fuel ratio sensor (this is what Toyota calls the "oxygen sensor"-like device on the exhaust manifold)

I replaced it, and didn't have any problems until last week.

The check engine light came on again, with the same error code (too lean).

What could cause the air/fuel ratio to go bad so quickly?

Or, does the error message imply something else is at fault?

Thanks for any help,

Sabbates