Toyota Repair: 01 Toyota Tacoma 2WD BAD gas mileage, gas mileage, first tank

Question
QUESTION: I purchased a used 2001 Tacoma 2WD Ext cab a few months ago. 13 gallon tank i believe, and I'm getting 150 - 160 miles per TANK! Air filter and plugs have been replaced. Seems to run a little better, but gas mileage is still the same. I've been told O2 Sensor. Checked for vaccuum leak, but was told there isnt one. Im pretty desperate at this point, PLEASE help with any suggestions.....thanks!

ANSWER: Does the check engine light turn on while driving?
Does the check engine light turn on with the key on?

---------- FOLLOW-UP ----------

QUESTION: Check engine light was on when purchased as is. They agreed to look at it and take care of what was wrong. When I picked up the truck the check engine light wasnt on anymore. I didnt think anything else about it,filled up the tank and went. When I got 152 miles out of the first tank, I had it put on a diagnostic machine at a local auto parts place. It showed bad 02 Sensor and/or connection. It was then I realized the check engine light wasnt coming on when I turned the key to start it. Called place I bought it from. They agreed to replace 02 Sensor. Took it in. Picked it up, 157 miles out of the next full tank. Took it to my mechanic. He showed me the 02 Sensor and it hadnt been replaced! He changed plugs and all filters. Told me to run this tank and if gas mileage didnt improve that he would replace 02 Sensor. He said they originally had probably just disconnected the check engine light. Going to have the light checked out this week. Gas mileage no better this tank. Finding it hard to figure out who to trust. Any suggestions?

Answer
You sure can't trust those people that sold it to you, most likely they took the check engine light bulb out to cover up a problem and lied to you about the O2 sensor. What I would suggest at this time before having the light looked at is to go to an Autozone or similar parts store and have them connect a diagnostic tool to get the trouble codes out of the computer, just give me the code numbers so I can make some sense of this.