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DTC codes / P00xx · Fuel and air metering and auxiliary emission controls / P004D

P004DTurbocharger/Supercharger Boost Control B Circuit High

P004D indicates that the powertrain control module (PCM) has detected an abnormally high voltage on the 'B' circuit that controls the turbocharger or supercharger boost control solenoid or actuator. This high voltage reading suggests an open circuit, a short to voltage, or a faulty component within that specific control circuit, which can prevent the boost control system from functioning correctly.

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, often with no other noticeable symptoms.
  • Reduced engine performance, such as a lack of power or sluggish acceleration due to the boost control system being disabled.
  • The engine may enter limp mode, limiting speed and RPM to protect the turbocharger or engine.
  • Poor fuel economy or increased exhaust smoke if the turbocharger is not boosting as commanded.
  • In some cases, the engine may run rough or hesitate under load if the boost control is stuck in one position.
  • A noticeable whistle or unusual noise from the turbocharger area if the actuator is stuck open or closed.

Common causes, most likely first

  1. A faulty boost control solenoid or actuator (the 'B' circuit component) that has failed internally and is causing a high voltage signal.
  2. Damaged or corroded wiring in the boost control 'B' circuit, such as an open circuit or a short to voltage.
  3. A poor electrical connection at the boost control solenoid or at the PCM connector, leading to high resistance or intermittent voltage spikes.
  4. A failed PCM driver for the boost control circuit, though this is less common and should be diagnosed last.
  5. Aftermarket modifications or poor previous repairs to the boost control wiring that have introduced a high voltage condition.
  6. A short to voltage in a related circuit that shares a common power or ground path with the boost control 'B' circuit.

How to diagnose and fix it

  1. Perform a visual inspection of the boost control solenoid, actuator, and all associated wiring for signs of damage, corrosion, or loose connections.
  2. Use a scan tool to check for any related diagnostic trouble codes (DTCs) and to view live data for the boost control circuit voltage to confirm the high voltage condition.
  3. Disconnect the boost control solenoid connector and measure the voltage on the signal circuit with the key on; compare to manufacturer specifications to determine if the high voltage is present at the solenoid or upstream.
  4. Check the resistance of the boost control solenoid or actuator according to the service manual and replace it if it is out of specification.
  5. Inspect the wiring harness for opens or shorts to voltage, using a multimeter to check continuity and voltage drop across the circuit; repair any damaged wires.
  6. If the wiring and solenoid are good, test the PCM driver circuit with a noid light or oscilloscope to verify proper operation; replace the PCM only if all other possibilities are ruled out.
  7. Clear the DTC and perform a road test to verify the repair, ensuring the boost control system operates correctly and the code does not return.

Boost control circuit B high voltage pitfalls

  • A common misdiagnosis is replacing the turbocharger or supercharger itself when the actual fault is a simple wiring issue or a failed solenoid, so always test the circuit before condemning expensive components.
  • The 'B' circuit designation often refers to a specific control channel for a variable geometry turbo or a second stage of a compound turbo system, so consult the service manual for the exact circuit location on your vehicle.
  • High voltage on a control circuit can sometimes be caused by a faulty ground connection elsewhere in the system, leading to a voltage reference shift that affects multiple sensors or actuators.
  • Intermittent high voltage codes can be caused by chafed wiring that only shorts to voltage when the engine moves under load, so wiggle testing the harness while monitoring live data is a valuable diagnostic step.
  • Some vehicles use a PWM signal for boost control, and a high voltage code may set if the duty cycle is outside the expected range due to a failed driver in the PCM, which requires advanced diagnostics to confirm.

Can I keep driving with P004D?

It is generally safe to drive the vehicle with this code as long as the engine is not in limp mode and performance is not severely degraded, but avoid aggressive acceleration or heavy towing until the fault is repaired. Prolonged driving with a boost control fault can lead to poor fuel economy and increased emissions, and in some cases may cause damage to the turbocharger if the boost is uncontrolled.

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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