I have a 2000 Nissan Xterra 4WD with a bad idle issue. Last week this customer dropped off his car complaining his check engine light was on and was around 1400 RPM at idle. Came up with 2 codes, P0325 Knock sensor malfunction and I think it was P0128 Idle Air Control valve malfunction. Customer said he never changed those parts in the history of owning the car and was the original owner. I suggested to him to change out his IAC which is under the upper intake toward firewall and the knock sensor under his lower intake. The work went swift but the all hell broke loose. Car at idle surges from 1200-2000 RPM even after warmup and the knock sensor code came back. Worst part is vehicle now needs inspection and cannot for the life of me fix the idle or knock sensor. We used all dealer parts during the work. Dealer told me that the knock sensor code is common on alot of Nissans and has something to do with the harness. The kicker of all of this is that before we did the work the idle never surged it was just higher then normal, got any advice.