Code
P1325
SUBARU
P — Powertrain
Knock Sensor Circuit Low Input
AI status
Completed
Completed
100%
Causes
- Failed knock sensor (piezo element open or degraded)
- Wiring short to ground or damaged wiring harness between knock sensor and ECM
- Corroded/loose connector or poor pin contact at sensor or ECM
- Poor engine block ground or grounding strap issues
- ECM input circuit fault or internal ECU failure
- Incorrect or aftermarket knock sensor with wrong characteristics
Symptoms
- Check Engine Light illuminated (MIL)
- Reduced engine power or torque management / limp-in behavior on some models
- Degraded acceleration, hesitation or stumbling under load
- Unusual pinging or detonation symptoms (if knock control disabled)
- Possible poor fuel economy
What to check
- Read freeze frame and pending/related codes; confirm P1325 is current
- Scan live data for knock sensor counts/voltage and related sensor data while cranking/under load
- Visually inspect sensor, connector, and harness for damage, corrosion, or oil/water intrusion
- Wiggle test wiring while monitoring live data or continuity to reproduce fault
- Measure continuity and resistance of harness to ECM (check for short to ground or open)
- Back-probe sensor connector and verify signal with a DVOM and oscilloscope if available
Signal parameters
- Sensor type: passive piezoelectric (no steady DC supply) — generates AC pulses when subjected to vibration/knock
- Expected behavior: low/no signal at idle with no knock; measurable AC pulses (millivolt to volt range) when knock is present or when the block is tapped during test
- A constant low/near-zero voltage or no AC activity when knock is induced indicates low input or open/short condition
- High impedance between sensor and ECU; continuity to ECU pin required, no short to ground
Diagnostic algorithm
- Confirm code and clear codes. Re-run to see if P1325 returns and note freeze frame/conditions.
- Visually inspect the knock sensor, mounting area, and connector for damage, oil or corrosion. Repair or replace any damaged hardware.
- With ignition OFF, unplug the sensor connector. Check continuity between the sensor terminal and the corresponding ECM pin; verify there is not a short to ground (unless sensor uses ground reference through block).
- If the sensor has measurable resistance spec available, measure sensor resistance. If open or out of spec, replace sensor.
- Back-probe the sensor connector with ignition ON (engine OFF). Use a scope to observe AC waveform while lightly tapping the engine block near the sensor or while performing a controlled rev/load test. Look for measurable AC pulses when the block is tapped.
- Wiggle harness and connectors while monitoring live data/oscilloscope to find intermittent faults. Repair any chafing or broken wires and secure harness away from heat/moving parts.
- Verify and clean engine ground(s) and the ECM connector ground. Repair as necessary.
- If wiring and sensor check good but no signal, consider replacing sensor. After replacement, clear codes and perform a road/test under load to confirm the code does not return.
- If fault persists after sensor and wiring replacement, consider ECM input circuit testing or replacement — consult manufacturer procedures before replacing ECM.
Likely causes
- Damaged/shorted wiring or connector between knock sensor and ECU
- Failed knock sensor
- Poor sensor ground/engine ground
- Corroded connector pin or water intrusion at sensor connector
Fault status
Status
Knock sensor circuit low input detected — sensor output below expected level (possible short to ground, open sensor, poor connector, or wiring fault).
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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