A P0328 trouble code indicates a high input from the knock sensor, often leading to reduced engine performance and fuel economy in Subaru Forester models.
Reviewed by SubaruReview Editorial Team · Last reviewed 2026-05-15 · Editorial standards
How this problem ranks against the other documented Forester failures — cost, severity, and DIY difficulty at a glance.
| Problem | Years | Severity | Repair cost | DIY |
|---|---|---|---|---|
| Knock Sensor Failure (P0328) (this page) | 1999-2008 | Major | $200–$400 | 4/5 |
| Subaru Forester Catalytic Converter Theft & Exhaust Damage | 2014-2022 | Major | $1,800–$3,500 | 3/5 |
This is one of several documented Forester problems we track, and it fits inside the broader Engine Guides guide. If you're weighing repair against replacement, cross-check the Subaru repair-cost database and any open NHTSA recalls on your VIN before booking a shop.
The knock sensor detects abnormal vibrations (knocking or pre-ignition) within the engine. When the sensor reports an abnormally high voltage, the Engine Control Unit (ECU) interprets this as severe engine knocking and triggers a P0328 code. This often causes the ECU to retard ignition timing significantly to protect the engine, leading to a noticeable decrease in power and fuel efficiency.
The most common cause is a faulty knock sensor itself, which can degrade over time due to heat cycling and vibrations. Wiring issues, such as a damaged harness or corroded connector, can also lead to an intermittent or constant high signal. In rare cases, severe actual engine knocking from other issues could trigger the code, but the sensor failing is far more common.
The most common cause is a faulty knock sensor itself, which can degrade over time due to heat cycling and vibrations. Wiring issues, such as a damaged harness or corroded connector, can also lead to an intermittent or constant high signal. In rare cases, severe actual engine knocking from other issues could trigger the code, but the sensor failing is far more common.
National average $200–$400.
Difficulty 4/5. Replace the faulty knock sensor with a new OEM or high-quality aftermarket unit.
Check Engine Light illuminated; Reduced engine power; Poor fuel economy; Engine hesitation or stumbling under acceleration; Engine pinging or knocking sounds (less common, as ECU retards timing)
Owners typically pay $200–$400 at an independent Subaru shop, parts and labor included. Our Subaru Timing Chain Replacement Cost (FB Engines) guide breaks the job down by parts, labor hours, and where the money actually goes.
The same generation also sees Subaru Forester Catalytic Converter Theft & Exhaust Damage. Both are documented with symptoms, cost, and a permanent-fix walkthrough — see the full list in the Forester hub.
We rate it 4/5 on our DIY scale. Most owners come out ahead with an independent Subaru shop; read labor-hour benchmarks in the repair-costs hub before you commit tools and a weekend.
Dig into the Problems Database to plan your next maintenance sprint, or browse every model hub for buyer's guides, generation breakdowns, and known-issue lists.