Clearing the C0042 diagnostic trouble code (DTC) in Subaru vehicles involves addressing the underlying issue causing it, typically related to the Wheel Speed Sensor (WSS) circuit for the left rear wheel, before resetting. Mere code clearing without fixing the root cause will result in the code quickly returning. Below are valid methods:
Essential Diagnostic Checks First
Before attempting reset, perform these crucial checks:
- Verify Tire Pressures: Ensure all tires, especially the left rear, are inflated to the exact manufacturer's specification. Mismatched pressures trigger this code.
- Inspect Wheel Speed Sensor & Wiring: Visually check the left rear WSS and its wiring harness for physical damage, corrosion, contamination (metal shavings), or loose connections at the sensor and ABS module.
- Check Wheel Bearing Play: Excessive play or damage in the left rear wheel bearing can affect the sensor reluctor ring.
- Examine Sensor Reluctor Ring (if accessible): Look for damage, cracks, or debris buildup on the ring integrated with the wheel bearing/hub assembly.
Methods to Clear/Reset C0042 Code
Once root causes are addressed or for interim clearing after repair:
1. Using an OBD-II Scan Tool (Recommended & Safest):
- Connect a compatible OBD-II scanner to the port under the dash.
- Turn ignition to ON (engine off).
- Navigate to the menu to read ABS/VDC DTCs (C0042 will be present).
- Select the option to Clear DTCs/Codes within the ABS/VDC module menu specifically.
- Confirm clearance. Turn ignition OFF for 30 seconds, then restart the vehicle. Verify code is gone and ABS/VDC warning lights are off.
2. Disconnecting the Battery (Temporary/Secondary Method):
- Turn ignition OFF and remove key.
- Disconnect the negative (-) battery terminal.
- Wait at least 15-30 minutes (longer ensures module capacitors discharge).
- Reconnect the negative battery terminal securely.
- Turn ignition to ON (do not start). ABS/VDC lights may illuminate initially.
- Start the engine and drive the vehicle above 20-30 mph for a few minutes. This allows systems to reinitialize. The code may clear if the issue was temporary/low voltage. Warning: Resets other learned parameters (radio presets, etc.).
3. Driving Cycle Method (Potential Passive Clear):
- Fix the root cause (e.g., correct tire pressure, repair sensor wiring).
- Ensure the battery voltage is optimal.
- Drive the vehicle through multiple drive cycles (cold start, normal driving, key-off periods). After 3-5 consecutive successful drive cycles without the fault reoccurring, the ECU may automatically clear the stored code.
Critical Precautions
- DO NOT simply clear the code with a scanner without diagnosing and fixing the underlying problem. The ABS, VDC, and potentially Vehicle Dynamics Control systems may remain disabled, compromising safety.
- Clearing the code does not fix mechanical or electrical faults. Permanently resolving C0042 requires correcting the root issue identified during diagnostics.
- If using the battery disconnect method, the ABS/VDC systems will be inactive until you drive above the relearn speed threshold. Drive cautiously.