97 century EGR flow insufficient code problem

I have 1997 Century with "EGR flow insufficient" code when "service engine soon" light came on. Car has 172000 miles.

My mechanic replaced the "EGR valve/gasket" with a brand new one.

After 120 miles, the same code came up. The mechanic now thinks that there is carbon build up in the intake manifold that needs cleaning. (He also suggested that "may be" the catalytic converter is somewhat plugged up - the converter is original, never replaced.)

Does any one have any other suggestions?

Thanks.

These maniflolds are known to plug up in the EGR passages. Have it cleaned out.