Check engine light BACK on ('95 Land Cruiser)

So, my check engine light came on on my '95 TLC. My mechanic said the
codes showed a failing #1 oxy sensor. I replaced it and pulled the EFI
fuse to reset the computer... after a few days the light came back on.
I went meekly back to my mechanic who checked again, saw the codes for
the #1 sensor, checked the new sensor's performance ("perfect, fast")
and reset the computer with his tool. About a week later, the light is
back on. He said it's throwing the code for that sensor plus the code
for a general lean condition, which usually goes hand in hand with the
02 sensor...

Any ideas, before I take it in again?

-jeff