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

Any ideas, before I take it in again?

-jeff