P013c -The check engine light after 65mph for 10 miles

Question

An oxygen sensor was replaced. The check engin light came on when traveling at65+ mph for 10 miles or more. Dealer could find no problem and reset the light. drove for about a week at city driving conditions with no problem. First time driving at 65+mph light came on and code was P013C. Dealer said if it happened again they may have to replace the foward converter?