03 MPV LX engine code knock sensor

My room mate has a 2003 Mazda MPV LX that suddenly started running rough, and threw engine codes. I can't find the paper with the specific codes, but it was the lean fuel mixture, and engine knock sensor codes. Friend lost his job recently so money is tight. Any one know what this could be? Do we need to replace the sensor? Where is the Knock Sensor located? Thanks for your help!!!

Don't know the exact code, but on our 2001 MPV, it was running rough, and the check engine light started flashing. The mechanic said it was the ignition coil. We had already had P0481 and P0051, which I determined to be the Oxygen sensor for bank 2. To fix the sensor and the misfire, it was around $550, which is flooring us. I'll get the exact code, if it helps.