2001 Nissan Xterra Check Engine Light On

2001 Nissan Xterra - the service engine soon light is on. Codes read at AutoZone said knock sensor. Took to our mechanic and he said it wasn't the knock sensor. He replaced the electronic cellinoid sensor (for cooling). He could not find an O2 sensor code for a 2001 Xterra. After driving 15 miles or so, the light is back on. We just need to get this fixed (as cheaply as possible) so we can sell this vehicle as we bought a new one. We've already paid for the coolant sensor and that did not fix the problem.