2002 Mercury Cougar service engine light egr code

Well I need to know what the P code is before I can be of much help. As for the tank having pressure not alway's true. My 95 F-150 no pressure when I remove the cap and my 98 Mustang same no pressure. So that is not alway's a tell tale sign ok. So if you would first get back to me with the code so I can look it up and see what all may be related to it and we will go from there.

code is p401 insufficent flow

Ok this code is a little bit generic. It if insufficent flow but this could be for any number of reasons that it if setting this code. The egr system works with many other sensors. It works with the engine coolant temp sensor (ECT), the air intake temp sensor (IAT), throttle position sensor (TPS), mass air flow sensor (MAF), and the crank position sensor (CKP). The PCM uses all these sensors to determine how much egr flow is needed. So I would like to test these one at a time to determine if one or more of these sensor are bad. You will need a digital multi meter in order to do these tests. First I would like you to test the ECT and the IAT sensors. You simply unplug them and check the ohms of resistance of the sensors (not the wire harness). If you do this when the car is cold then get back to me with the ohms and the outside air temp at the time you tested them. Lets start here as these do go bad more frequently then the rest. You also may need to get some mass air flow cleaner (no other type will work) and clean the mass air flow meter. Anyway lets start with those sensors. Get back to me with what you find and we will take it from there. Here is a pic of the ECT first and then the IAT sensor.

IAT Sensor