P0325 is a powertrain OBDII trouble code that has the same meaning for the Mercury Mariner as it would any other vehicle. By definition, it means that the vehicles ECM/PCM isn’t getting the proper signal to or from the knock sensor. Clearing this code typically requires a new knock sensor or correcting a short or break in the wiring harness.
Modern internal combustion engines utilize the knock sensor to allow for aggressive timing. If the timing has gotten too aggressive it’ll cause the engine to knock. This sensor will let the computer know to dial back the timing.
P0325 Symptoms: Mercury Mariner
The symptoms associated with P0325 are going to vary by engine and model year. Without a signal from the knock sensor, many engines will curve the timing and power output to make sure that the engine is not knocking. The only other symptom that you’ll probably notice is the service engine soon light itself.
P0325 Causes: Mercury Mariner
Most of the time, the fix for P0325 in the Mariner (or most any vehicle) is going to come from replacing the knock sensor or repairing the wiring harness.
Wiring Harness
P0325 is telling you that the ECM/PCM is not getting the proper signal from the knock sensor. When this happens the computer will operate as though it is getting no signal from the sensor at all. A good place to start would be to check the wiring leading from the sensor and see if any of it has been damaged or frayed. Most of them bolt right into the block, which makes accessing them difficult. Their proximity to the engine block means that they are often susceptible to damage.
Checking Car Wiring Circuits For Continuity with a Multimeter (Youtube)
How To Find a Electrical Short On Most Any Vehicle (Backyard Mechanic)
Knock Sensor
If you find that the wiring harness is ok, and the code is still persistently registering, it is time to test and/or replace the knock sensor. There are guides out there on how to test a knock sensor, but they are very affordable. They can also take a little digging to get to, which makes replacing it the way to go.
How to Test a Knock Sensor (It Still Runs)
Knock Sensor Mercury Mariner (AMAZON)
Conclusion
The most difficult part in fixing your Mariner’s engine to clear the P0325 code is going to be finding room to maneuver and inspect the Knock Sensor and wiring harness. This can be tedious work. Good luck!