I am facing strange problems with my 2000 NISSAN Altima GXE. My car was showing Check Engine light with codes of Front Oxygen sensor and Knock sensor. After repair it started showing code for EGR valve and check engine sign came again. After getting new EGR valve now it started showing new code P1400 (EGRC Solenoid Valve). I already spent more than $400 and codes are still coming 1 by 1. I don't know why these codes don't come same time but 1 after another. I am not able to find the part (EGRC Solenoid Valve) online and at any store. Please give me any idea how much this part cost and how much will be labor cost. Also can I test/repair it myself??