My 2000 F-150 has a check engine light on. I think it was code p1405. The EGR valve pressure switch. Will this affect my miles per gallon?

 

Usually anytime you have a check engine light on the vehicle basically goes into a limp mode whether its noticeable or not and it can affect your gas mileage.

 

Yes and your evap system probably. Check your vacuum lines and clamps for leaks. Especially to valve. Run OBD II codes , freeze frames and I/M tests. Document all your pages befor reseting/clearing codes. Follow the leads the reader gives you and the tests that don't pass. You can't shut off the MIL and keep it off until all codes are cleared and repair made.