Q: PO400 code came up with multiple cylinder misfires. I replaced the EGR solenoid, all the vacuum lines, cleaned the EGR pipe and passage. Still have the same problem.

Hi there. After having the EGR (engine gas recirculation) valve replaced, the vacuum lines replaced, and the tubes cleaned out, then there could be an electrical issue with the EGR system. Check the EGR harness with a digital meter and see if there’s power to the EGR valve. If there is power, then check the other contact and see if there is a continuity to the computer system. If there is no continuity, then the wires would be broken and would need repaired or the harness would need replaced. If the wiring is in good condition but the code keeps coming up with the EGR valve, then either the new EGR valve is defective or the computer (ECM) has a burned contact or has a defect and would need replaced. If you need further assistance with your engine misfiring due to the EGR system, then seek out a professional, such as one from Your Mechanic, to help you.