What it means
The ECU sees the control voltage of fuel volume regulator A above the expected range, typically a short to power or open ground return.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
The fuel volume regulator meters how much fuel the pump supplies, and the ECU controls it through regulator A's electrical circuit. P0004 sets when the ECU detects that circuit's control voltage above its expected range, which can happen if the wiring is shorted to power or the ground return is open.
Common causes
Most likely first.
- Wiring shorted to battery voltage
- Open ground return path
- Connector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Hard starting or no start
- Engine may stall or run poorly
- Reduced power
How to diagnose it
- Record freeze-frame data and check fuel-pressure and regulator command readings to see whether the fault appeared during starting, idling, or load.
- Inspect regulator A's connector and harness for corrosion, loose or backed-out pins, chafing, or contact with a power source.
- With the circuit safely isolated as specified by the manufacturer, check for an unintended short to power and verify continuity of the ground return.
- Compare voltage at the regulator and ECU terminals with the manufacturer's specification while commanding the regulator, if the scan tool supports it.
- If wiring and connections pass, test regulator A's coil and operation to determine whether the regulator itself is faulty.
Common fixes
- Clean and secure corroded or loose regulator A connector terminals.
- Repair wiring shorted to power or restore the open ground return.
- Replace damaged connector pins or harness sections.
- Replace fuel volume regulator A if its electrical or operating tests fail.
Can I keep driving?
Hard starting, stalling, or reduced power can make driving unpredictable, so avoid traffic and high-load driving. If the engine stalls or will not start reliably, stop driving and arrange recovery.
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.