What it means
The battery charger A input voltage sensor B circuit signal is below the expected threshold. The code does not by itself identify the cause.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Battery charger A uses input voltage sensor B to report an electrical condition at the charger input to the control module. The module sets P0EF9 when sensor B's circuit signal falls below its expected range, which can make it misjudge charging conditions.
Common causes
Most likely first.
- Signal wire shorted to ground
- Internal sensor short
- Connector pins corroded - high contact resistance
Symptoms
- Charge status indicator shows fault
- Charging may not start or interrupts
- Slower-than-expected charge rate
How to diagnose it
- Record freeze-frame data and compare sensor B's live reading with the charger input readings to see when the signal drops low.
- With the vehicle safely powered down according to the manufacturer's procedure, inspect sensor B's connector and wiring for damage, corrosion, moisture, or a wire contacting ground.
- Check the connector pins for corrosion or poor contact that could raise resistance and pull the signal down.
- Test sensor B's signal wire for a short to ground and check continuity through the circuit using the manufacturer's procedure.
- If wiring and connections pass, test sensor B and its ground/reference circuits to determine whether the sensor itself is pulling the signal low.
Common fixes
- Clean and securely reconnect corroded or loose sensor B connector pins.
- Repair a signal wire shorted to ground or damaged wiring.
- Restore a poor sensor ground or reference connection.
- Replace input voltage sensor B if testing confirms an internal short.
Can I keep driving?
Charging may fail to start, stop unexpectedly, or proceed more slowly, and the vehicle may limit charging functions. Avoid relying on normal charging until the fault is diagnosed; follow the vehicle's warnings and charging safety instructions.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.