2001 Altima Check Engine Light P0240

I have a 2001 Nissan Altima GXE (2.4) with 109,000 miles. I am receiving two error codes. One of the codes was for a misfire on 4 which was solved by replacing the intake manifold gasket. The other code was P0240. I have already replaced both O2 sensors and the catalytic converter has also been replaced. After I reset the system, the check engine light comes on again somewhere between 15-30 miles after resetting it. What else should I look for that could be causing the problem? JTSHONO@yahoo. Com