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DTC codes / P00xx · Fuel and air metering and auxiliary emission controls / P004C

P004CTurbocharger/Supercharger Boost Control B Circuit Low

P004C indicates that the powertrain control module (PCM) has detected an abnormally low voltage or current on the 'B' circuit that controls the turbocharger or supercharger boost control solenoid or actuator. This circuit is responsible for regulating boost pressure, and a low signal means the PCM cannot properly command the boost control device, potentially leading to reduced engine performance and increased emissions.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light (malfunction indicator lamp) on the dashboard.
  • Reduced engine power or limp mode, often noticeable during acceleration.
  • Poor fuel economy due to the engine running in a default boost strategy.
  • Audible hissing or whistling from the turbocharger area if the wastegate or bypass valve is stuck open.
  • Hesitation or stumble when accelerating, especially under load.
  • Often no symptoms besides the warning light, particularly if the fault is intermittent.

Common causes, most likely first

  1. Damaged, corroded, or disconnected wiring or connector at the boost control solenoid or actuator.
  2. Failed boost control solenoid or actuator (open or shorted internally).
  3. Blown fuse or poor ground connection supplying power to the boost control circuit.
  4. Vacuum or boost leak in the hoses connected to the wastegate or bypass valve, causing the solenoid to work outside its expected range.
  5. Sticking or seized wastegate or bypass valve linkage, preventing proper movement and causing the solenoid to draw abnormal current.
  6. Faulty powertrain control module (PCM) driver circuit, though this is rare.

How to diagnose and fix it

  1. Perform a visual inspection of the boost control solenoid, actuator, and all related wiring and connectors for damage, corrosion, or looseness.
  2. Check the fuse and ground connections for the boost control circuit using a multimeter to verify power and ground.
  3. Disconnect the boost control solenoid and measure its resistance; compare to the manufacturer's specification to rule out an open or shorted coil.
  4. Inspect all vacuum and boost hoses for cracks, splits, or disconnections, and repair or replace as needed.
  5. Use a scan tool to command the boost control solenoid on and off while listening for a click and observing actuator movement; lack of response indicates a faulty solenoid or actuator.
  6. Backprobe the signal wire at the PCM and measure voltage while commanding the solenoid; if voltage is low when it should be high, check for a short to ground or an open circuit in the wire.
  7. If all electrical and mechanical checks pass, suspect a faulty PCM, but only after confirming all other possibilities and consulting factory diagnostic procedures.

Common misdiagnosis traps for P004C

  • A sticking wastegate or bypass valve can cause the boost control solenoid to draw excessive current, setting a low circuit code even though the solenoid itself is good.
  • Corroded pins in the boost control solenoid connector are a frequent cause that is easily overlooked during visual inspection.
  • The code can be set by a vacuum leak that causes the solenoid to operate outside its expected duty cycle, so always check hoses before replacing parts.
  • In some vehicles, the boost control circuit is shared with other solenoids, and a fault in a related system can trigger P004C as a secondary code.
  • Intermittent faults may only appear under specific conditions such as cold starts or high load, so a wiggle test of the harness while monitoring live data is essential.

Can I keep driving with P004C?

You can usually drive the vehicle with P004C, but expect reduced power and possible limp mode; avoid heavy acceleration or towing until the fault is repaired to prevent further damage to the turbocharger or engine.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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