Poor fuel economy

I have a 2001 Sienna XLE, V-6, 3.0 l with 90K miles. Recently, the "check engine light" came on and a P-420 code was recorded. The dealer suggested replacement of the computer. He said until the computer is replaced, the gas mile will go down, which it did. Last week, the computer was replaced and on the first tank of gas, the fuel economy does not seem to be any better than before the computer replacement. Any suggestions?