I have a 95 F150, 4.9 5 speed. I get abot 9mpg. New exhaust, new cat, egr, air charge sensor, temp cooant sensor. Still have the bad mileage. I changed the plugs on Sunday and started getting a CEl on tuesday. When I took it to the mechanic and when he hooked up his scanner it will not go into self diagnose and says there is no codes even thought the CEL light is on. Any ideas.

 

Check the Fuel Pressure. At idle, it should be around 55PSI.

Another thing could be the O2 sensor...but since no codes are showing up, Its hard to tell.

 

If the light is on, there should be codes. If he can't pull them either he's not doing it right or the computer's fried.