Understanding the P1420 Code on Subaru Vehicles
The OBD2 P1420 code is a manufacturer-specific code for Subaru that indicates a malfunction in the wastegate control valve circuit. This system is crucial for regulating boost pressure in Subaru turbocharged engines. When the engine control unit (ECU) detects an anomaly in this circuit, it records the P1420 code and illuminates the malfunction indicator lamp (MIL) on your dashboard.
Role of the Wastegate in the Subaru Turbo Engine
The wastegate, also known as the discharge valve, is an essential component of the turbocharger system. Its main functions are to:
- Control boost pressure to prevent engine damage
- Divert excess exhaust gases when maximum pressure is reached
- Maintain optimal turbocharger performance
- Protect the engine from dangerous overpressure conditions
Operation of the Electric Control Circuit
Unlike purely pneumatic systems, the ECU-controlled circuit in Subaru uses an electric actuator or solenoid valve that:
- Receives PWM (Pulse Width Modulation) signals from the computer
- Converts these signals into mechanical action to open/close the valve
- Transmits feedback data to the computer for verification
Diagnosing the P1420 Code: Step-by-Step Methodology
Diagnosing the P1420 code requires a systematic approach to accurately identify the source of the problem. Here is the procedure recommended by Subaru experts.
Equipment Needed for Diagnosis
- Subaru-compatible OBD2 scanner with real-time data reading capabilities
- Quality digital multimeter with ohmmeter and continuity test functions
- Standard mechanical tools to access components
- Technical documentation of electrical schematics specific to the model
- Vacuum pump to test mechanical operation of the valve
Electrical Circuit Verification Procedure
Start with a visual inspection of the wiring and connectors:
- Check the condition of the wastegate valve and computer connectors
- Look for damaged, chafing, or burned wires
- Check the circuit’s power supply voltage and ground
- Test the resistance of the actuator coil (typically between 10-30 ohms)
- Verify continuity between the ECU and the wastegate valve
Testing Wastegate Valve Operation
To test the mechanical component itself:
- Disconnect the electrical connector from the valve
- Apply a compressed air source or vacuum pump according to the valve type
- Verify that the valve moves freely without blockage
- Check the seal of the diaphragm or piston
- Test the return spring to ensure proper operation
Common Causes and Solutions for P1420
Several elements can trigger the P1420 code. Here are the most common causes listed in order of probability.
Electrical and Wiring Problems
- Oxidized or loose connectors: Cleaning or replacement of connectors
- Damaged wiring: Repair or replacement of faulty wires
- Ground issues: Verification and cleaning of ground points
- Short circuit or open circuit: Locating and repairing electrical faults
Wastegate Valve Failures
- Mechanically stuck valve: Cleaning or replacement of the valve
- Torn diaphragm: Replacement of the complete unit
- Defective electric coil: Resistance measurement and replacement if necessary
- Normal component wear: Preventive replacement after verification
Engine Control Unit (ECU) Problems
Although less frequent, the ECU may be the cause:
- Internal failure of power drivers
- CAN bus communication problems
- Corrupted software requiring an update
- Damage caused by electrical surges
Consequences and Risks of Driving with an Active P1420
Driving with an active P1420 code presents several risks that are important to understand.
Impact on Engine Performance
- Significant reduction in engine power (limp mode)
- Increased fuel consumption
- Delayed turbocharger response
- Risk of turbocharger overheating
Risks of Mechanical Damage
- Overboost conditions that can damage the turbocharger
- Premature deterioration of engine components
- Increased pollutant emissions
- Accelerated wear of the exhaust system
Reset Procedures and Repair Validation
After performing repairs, it is essential to follow a rigorous validation procedure.
Code Clearing and Drive Cycle
- Use your OBD2 scanner to clear the fault code
- Perform a complete drive cycle to allow OBD2 monitors to reset
- Verify that the code does not reappear after several ignition cycles
- Check that all monitoring systems return to “Ready” status
Repair Validation Tests
- Performance test under load to verify boost pressure
- Real-time parameter monitoring with OBD2 scanner
- Verification of no pending or permanent codes
- Check of normal turbocharger operation across the entire RPM range
The P1420 code on Subaru requires a methodical diagnostic approach but remains perfectly repairable with the right technical skills. Prompt resolution of this issue will preserve your Subaru’s performance and prevent more costly long-term damage.