Few months ago I received a code for the EGRC Solenoid Valve, then a week ago I received a code for the EGR Hi Flow Low system, and Front right O2 sensor. The EGRC Solenoid valve code never returned, nor did the EGR hi flow low system. How do I replace the valve, how do I know if it's bad? What should I expect to pay to have my EGR system cleaned out since I'm guessing it's built up with carbon '97 with 130K miles, and why won't the codes return if they came up in the first place many days ago?