I have a 2001 mercury sable 3.0 it has a p1151 code and seems to be using alot of gas but I dont really know from what I was told that the car has sat for a year or so what or some steps I could check before going and spending a few hundred dollars on a o2 sensor Ive checked for vacum leaks by ear I ear no hissing and the exhaust mani seems not to be cracked or anything by a quick sight inspection

SOURCE: Problem with 2001 Mercury Sable I have a 2001

When you ask for help with your vehicle, I personally recommend that you mention WHICH 3.oL engine your vehicle is equipped with. I know that you probably did not know that there is two different ones. There is a 3.0L Dual Over-Head Cam (DOHC) engine (VIN Code "S"), and there is a 3.0L Over-Head Valve (OHV) engine (VIN Code "U").
Both of these engines are notorious for Mass Air Flow (MAF) sensors and vacuum leaks, however, these problems will cause problems on BOTH banks, not just one. You most likely have a "lazy" oxygen sensor. It just isn't switching like it should. The best way to diagnose this condition is to get the car on a scanner that can read sensor data and look to see what the downstream oxygen sensors are doing. If you have a lean code for the upstream sensor and you have high voltage readings (RICH or above 600 millivolts) on the downstream sensor, then the engine is not really running lean - the upstream sensor is lying and needs to be replaced.

Since you are getting a lean oxygen sensor code and it is using a lot of gas, I would say that this is most likely the case. An oxygen sensor that is lying to the computer about the system being lean will cause the computer to open the injectors wide open. Take a look at your tailpipe and see if it is really black. A normal exhaust pipe will look light tan in color. If the tail pipe is black, this is a really good indication that the system is actually running rich and the oxygen sensor is lying about it.