1999 Mercury tracer code p0401

Answer 1

insufficient egr flow.

60%of Mercury's do this with a bad dpfe sensor. replace if for good measure, it's bound to go out some time even if it is not right now. you'll need to buy soem hoses for it too, the replacement dpfe isn't shaped the smae as the old one, and requires an extra inch or 2 on the hose to the ref port (you'll see, it's labeled that way on the sensor)

30% are because of blocked passages in the egr system. they get full of carbon and soot, and it probably needs cleaned out. you may have to take off your throttle body to find it, it depends on the engine.