The 2020 Subaru WRX uses the FA20DIT 2.0L turbocharged Boxer engine producing 268 horsepower. When knock is detected under boost along with P0325 (Knock Sensor 1 Circuit Malfunction), understanding whether this is a sensor issue or actual detonation is crucial for protecting the engine.
Understanding Engine Knock in the FA20
Knock (detonation or pre-ignition) occurs when the air-fuel mixture ignites before the spark plug fires, creating pressure waves that damage internal components. Turbocharged engines are particularly susceptible due to higher cylinder pressures from boost.
The WRX's knock sensors detect detonation and signal the ECU to retard timing, reducing knock but also power. If the knock sensor circuit fails (P0325), the ECU can't detect knock and may run unsafe timing.
P0325 Code Causes
Knock sensor failure is possible—sensors can fail internally or have damaged wiring. However, P0325 indicates a circuit issue, not necessarily knock detection. This could be the sensor, wiring harness, or ECU connection.
Wiring issues are common on Subaru Boxer engines due to sensor location. The knock sensors mount to the block in areas where wiring can chafe or corrode. Inspect wiring before condemning the sensor.
Actual Knock vs. Sensor Issues
If you're experiencing actual knock (audible pinging under boost), this is serious regardless of the P0325 code. Low-quality fuel, high intake air temperatures, and tune issues cause knock. Using 91+ octane fuel is essential for the turbocharged WRX.
If the engine runs fine but the code is present, the knock sensor or circuit may have failed. The engine should still be drivable, but knock protection is compromised. Repair promptly.
Diagnostic Approach
Check knock sensor wiring for damage, corrosion, or loose connections. Test sensor resistance with a multimeter per factory specifications. If the sensor tests good, check wiring continuity to the ECU. Consider whether any recent work might have disturbed the sensor or wiring.
Protecting the FA20
Always use premium fuel (91+ octane). Avoid aggressive driving when intake air temperatures are high. If you have an aftermarket tune, ensure it's from a reputable source with appropriate knock protection. Stock tunes are conservative but aftermarket tunes vary in knock margin.