P0328 on Subaru: How to Diagnose & Fix
Knock Sensor 1 Circuit High Input (Bank 1)
1 Subaru model · 1 documented issue · $80-$250 repair
P0328 on Subaru by model: symptoms, diagnosis and fix
The single knock sensor bolted to the top of the EJ block on SF- and early SG-generation Foresters has a brittle plastic/resin housing that cracks after years of heat cycling directly on the block. Once cracked, the piezo element's circuit reads out of range and the ECU sets P0325. The car keeps running, but the ECU falls back to a conservative safe ignition map: owners report noticeably reduced power, sluggish throttle response and a measurable drop in fuel economy. The failure is common enough that many independent shops replace the sensor on any high-mileage EJ Forester showing that code without further diagnosis. A secondary cause is the sensor wiring harness chafing or the connector corroding, so the harness should be inspected before condemning the sensor.
Common Symptoms
- Check engine light with P0325
- Reduced power and sluggish acceleration
- Worse fuel economy
- Engine feels flat under load
- Code returns shortly after a replacement sensor is overtightened
How to Fix
Replace the knock sensor — it sits on top of the block under the intake manifold and is reachable with a 12mm socket and extensions, typically about 30-60 minutes of work. Torque matters: overtightening cracks the new sensor's housing and brings the code straight back, so use the specified low torque value and do not impact it on. Inspect the harness pigtail and connector for green corrosion or chafe damage at the same time, and clear the code and confirm the ECU returns to normal timing. Parts are inexpensive. No recall or warranty extension applies.
Sources for this issue (3)
P0328 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0328 mean on a Subaru?
P0328 stands for "Knock Sensor 1 Circuit High Input (Bank 1)." This code means the knock sensor on engine bank 1 produced a voltage signal higher than its normal expected range. The knock sensor detects the vibrations of engine knock (detonation) so the computer can pull back ignition timing and protect the engine from damage. A high-input reading typically indicates an electrical issue such as a short to voltage or a wiring/connector fault, rather than genuine, sustained knock. While the sensor is faulted, the engine may default to conservative timing, slightly hurting performance and fuel economy. On Subaru it is documented across 1 model: Forester.
What are the most common causes of P0328?
The most common causes of P0328 are: Faulty knock sensor (Bank 1, Sensor 1), Sensor wiring shorted to voltage or power, Damaged or corroded sensor connector, Loose or improperly torqued sensor mounting, Chafed/pinched wiring harness, Grounding or shielding fault in the circuit, Faulty engine control module (PCM) — rare. The specific cause varies by vehicle.
How much does it cost to fix P0328 on a Subaru?
Published repair estimates on Subaru range from $80 to $250 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0328 on a different make?
View P0328 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.