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1999 Subaru Legacy GT (2.5L) OBD-II P0325 Code: Diagnosis, Fix & Emissions Test Tips

Model: 1999 Subaru Legacy GT (2.5L) Fault Code: P0325 Posted: 2008-06-16 09:20

My 1999 Subaru Legacy GT with a 2.5L engine has recently developed an intermittent Check Engine Light (CEL), now consistently showing OBD-II fault code P0325. The car has been maintained regularly and is under 150,000 miles. The issue began as intermittent — the CEL would turn on and off over several weeks — but during a recent emissions test, it activated after just 15 minutes of driving, resulting in an immediate failure. I noticed that when the CEL is on, the car runs sluggishly, especially uphill, with a slight fuel odor at low RPMs. There is no audible knocking detected. I've already checked all wiring connections and tightened the main engine block ground (large gauge wire from near the starter to battery negative). However, the P0325 code reappears every time the engine reaches normal operating temperature. The knock sensor is located on top of the engine block, about 6 inches below the throttle cable assembly. It appears as a rubber-covered donut with a bolt through it. Accessing it requires removing some components, but not the entire intake system. Research indicates that P0325 does not mean engine knocking — instead, it means the knock sensor output is outside expected values. The ECU then retards ignition timing and enriches fuel mix to enter failsafe mode, which explains reduced fuel efficiency. I’ve tested the sensor with a multimeter: when warm, resistance exceeded 20K ohms (spec is ~7–8K), suggesting a wiring or internal failure. My mechanic replaced the knock sensor, also replaced spark plugs that were over 75k miles old, and the car has since run smoothly. I’m now preparing for my emissions retest and would like to know: what constitutes a full driving cycle? I’ve seen conflicting advice — is it just mileage or a mix of highway and stop-and-go driving? I have an OBD-II scanner (a $99 tool with free shipping) that has been invaluable. I’m also considering using online resources like ALLDATAdiy.com for detailed repair guides, though subscriptions can be expensive. If anyone has experience with this specific code on a 1999 Subaru Legacy GT or similar models, any tips on diagnosis and access would be greatly appreciated.

Related fault codes
P0325
Comments (10)
Anonymous 2008-06-16 11:15

Check both the sensor circuit and ground connection for continuity, voltage, and proper grounding. Do you have a service manual? A Haynes or ALLDATAdiy guide can help with step-by-step access. Good luck — please update if you find anything!

Anonymous 2008-06-16 17:36

Same issue on my '99 Subaru Legacy GT (2.5L, 143K miles). The CEL activates when warmed up and clears after ~24 hours of inactivity. During emissions testing, it triggered after 15 minutes of operation — the test failed immediately due to P0325. I notice a noticeable drop in driveability on hills with the light on, and sometimes a faint gasoline smell at low RPMs. The ECU appears to be retarding timing and enriching fuel mix when the code is active, which likely explains poor fuel economy. The sensor sits about 6 inches below the throttle cable assembly, mounted on top of the engine block like a rubber donut with a bolt through it. My mechanic replaced the knock sensor and spark plugs (75K+ miles), and performance has improved significantly. I’ll share results soon — thanks for the insight!

Anonymous 2008-06-16 18:33

Thanks for sharing! I experience identical symptoms in my car — a sluggish feel on hills when CEL is on, with no knocking heard. The code clears after ~24 hours of non-use, just like yours. I don’t have a full service manual but plan to use ALLDATAdiy.com (though the subscription cost is high for one-off fixes). I own a Haynes manual — not very detailed on this component. I’ve tried poking at it when cold and will inspect wiring tonight. I have an OBD-II scanner, which has been extremely helpful with both of my Subarus. After resetting the code today and driving ~12 miles, it returned within minutes of reaching normal operating temperature. This supports the theory that the ECU is entering failsafe mode due to poor sensor feedback. I’m now checking harness connections and will update after tonight’s inspection.

Anonymous 2008-06-18 12:40

I have a 1998 Subaru Legacy with 217K miles and the same P0325 code. I cleared it and haven’t seen it return in over a week. I tried switching to higher octane fuel, but don’t know if that made a difference. The sensor is buried under wires and plumbing — access is tricky. It costs about $80 to replace, so I’m hesitant until I confirm the sensor is actually faulty. Let me know how your repair goes!

Anonymous 2008-06-19 21:36

My mechanic tested the knock sensor specs: expected impedance was 7–8K ohms. Mine showed resistance in the teens when cold, but over 20K ohms when warm — this likely explains the intermittent behavior. After replacing the sensor and upgrading spark plugs (75k miles), the car runs smoothly now. Emissions test passed today. Thanks for the help!

Anonymous 2008-06-20 14:05

The computer took longer than expected to initialize all sensors, but once complete, passing the emissions test was straightforward. Hope your vehicle passes as well!

Anonymous 2008-06-23 09:22

I returned to my car today and followed the diagnostic steps. First, I checked continuity from the sensor harness to engine block — found an open circuit! The wire had a break about 1.5 cm from the connector; insulation was brittle, likely due to heat exposure. After replacing the sensor, cleared the code, and now have driven over 100 miles with no CEL. The old sensor read higher than spec but still within acceptable range. Thanks for all the helpful input!

Anonymous 2008-06-23 09:33

Accessing the knock sensor on a 2.5L engine isn’t too difficult — just find it! Use a 12mm wrench to reach it straight down between the intake manifold and throttle cable assembly. I had to gently use a pressure sprayer (carefully!) to clear the top of the bolt head for visibility. One zip tie needs to be cut and replaced. Overall, it’s surprisingly straightforward once you locate it.

Anonymous 2008-06-24 17:48

CEL is now off, but I logged a 'No-Test' due to insufficient sensor initialization cycles. Question: How many miles or what driving pattern constitutes a full driving cycle? I couldn’t find this in any manuals — online resources don’t clarify whether it’s just mileage or requires specific mix of highway and stop-and-go driving. Any clues would be appreciated!

Anonymous 2008-06-25 12:39

I can't give a definitive answer, but I completed four trips totaling under 50 miles and passed the test. After picking up my car, I drove home about 10 miles on highway. The next morning, I took country roads for ~15 miles. First test failed due to incomplete sensor initialization — told that mix of stop-and-go and highway driving is needed. Then I made a 3-mile trip with multiple stops (about 30 minutes), followed by 12 miles back on the highway. The second attempt passed. This may be the key — hope it helps! Good luck!