Diagnostic trouble code P0325 indicates a circuit malfunction with knock sensor number one.

The knock sensor is used on engines to detect the vibrations from detonation, which is the pinging sound commonly known as spark knock.  The typical knock sensor is a piezoelectric device which is bolted or threaded into the block and generates a small voltage signal when it detects noise in the engine.  

To allow for wide variations in fuel quality and octane levels, the computer uses data from the knock sensor to retard ignition timing, when necessary, to allow the engine to run just under the point of fuel detonation for improved power and economy.

 

Causes for this code include:

Faulty knock sensor, knock sensor sub-harness, wiring problems between sensor and ECU, poor grounds.

 

Nissan OBD Code P0325

It's common for many of the Nissan models like the Altima, Maxima, and Quest (includes Mercury Villager) minivan to have a stored knock sensor code.  Since P0325 is so prevalent, usually causes no drivability concerns and will not set a "service engine soon" light on most Nissan models, it's widely ignored by technicians.  Reported fixes include genuine OEM knock sensor with sub-harness replacement.  Often the code is set with another code, which may be the underlying cause.  Observe proper torque specification when replacing knock sensor since a loose knock sensor can cause false codes.

 

Subaru OBD Code P0325 

Subaru, like Nissan, is known for their common P0325 knock sensor code.  But, unlike Nissan, knock sensor code P0325 on a Subaru will usually set a service engine light.  Sensors may test good, but still set the code.  Common fixes include OEM knock sensor replacement.  Replacement note: Clean corrosion from mounting surface and bolt threads.  Torque to recommended specification.