Poor Gas Mileage, No CE Light but many codes

[jkennedy3489]

I noticed that my highway mileage has dropped to about 35 mpg so I hooked up the handy OBD monitor and had 16 codes!! These included Mass Air Flow Circuit Malfunction, Mass Flow Low and High Input, Throttle switch malfunction, Engine coolant high input temperature, o2 heater malfunction, crankshaft position sensor circuit malfunction, and of course the good old assorted cylinder misfire (yeah). But the car still runs OK! Had a new mass flow so I changed it out for good measure with no improvement. Any idea what I should try next. I was kinda thinking about the Crankshaft or O2 sensor. Should be interesting thanks!