What causes a 1996 mercury villager odb codes p1400 p0135 p0325?

Answer 1

P1400 Differential Pressure Feedback EGR Circuit Low Input (Dealer TSB Number: 99103 addressed some possibles with this issue)

p0135 Heater O2 Sensor1 Bank 1 (This is the first sensor nearest the engine)

P0325 Knock Sensor

The last code often sets with almost any other problem, there is probably nothing wrong with the knock sensor itself, it's a passive part.

The O2 sensor may be bad, but it may also have a broken wire about six inches up the harness where there is a splice, this is a COMMON problem on these vehicles.

The P1400 I would not worry about until you get the O2 sensor problem fixed. These codes tend to daisy-chain on these vehicles, sometimes one problem will cause 3 codes, I've seen it many times.