Ford: Ford Trouble Codes, differential pressure sensor, mass air flow sensor

Question
I have a 1997 Ford F-150 Lariat, 4WD, 4.6l. Check engine light is on. I went to the local Autozone and had the code read. It came with 4 different codes. Ended up replacing the fuel cap, and the Mass Air Flow Sensor. Cleared the code. Light came back on and codes were P0455 and P0401. Where is the most common place to look for the problem? I have been tinkering around with cars for 20 years....Still not sure I like the new ones.....Please any help is appreciated

Answer
Hello,

P0401 is an EGR code.  Most common cause is a failed EGR differential pressure sensor.
this sensor is on left side of manifold and has 2 different size silicone hoses (gray or black) to 2 nipples on the exhaust and is used to determine if the egr is opening and how much.
If the hoses are good, then the sensor has most likely failed.

The P0455 indicates a small evaporative leak. somewhere between the purge valve (engine bay) and the gas cap.  Aftermarket caps can fail, as can cannisters, purge valve,sensor or other hoses.
a smoke machine is best for finding these leaks ( as small as .020") if there isn't a visible fault (ie: broken fitting or disconnected hose)