The P0637 code in your 2020 Subaru Forester indicates high voltage in the power steering control circuit. This is the opposite of P0636—instead of low voltage, the circuit is showing elevated voltage that exceeds normal operating parameters. This typically indicates an open circuit condition where current isn't flowing properly.
High Voltage Circuit Analysis
When a circuit is open or has very high resistance, minimal current flows. Voltage at measurement points appears higher than normal because there's no load pulling voltage down. The ECM detects this elevated voltage and triggers P0637, indicating the power steering communication circuit isn't completing properly.
Causes of High Circuit Voltage
An open circuit in the communication wiring prevents current flow, causing high voltage readings. Disconnected connectors or terminals create opens. The EPS module's communication input may have failed open internally. The ECM's communication output may have failed in a way that prevents proper current sink.
Symptoms in Your Forester
Power steering warning light illuminates, often with the check engine light. Steering assist may be completely absent if communication is lost. The EPS system may enter a fault mode showing limited or no functionality. Symptoms don't improve with vehicle warm-up if the circuit is truly open.
Subaru-Specific Diagnosis
Using Subaru SSM diagnostic software, check for communication between the ECM and EPS modules. High voltage readings on the communication circuit confirm the open condition. Test wiring continuity from ECM to EPS module. Inspect connectors for backed-out pins, corrosion, or damage. If communication occurs intermittently, look for loose connections that may not be obviously broken.