Technical Description of OBD-II Trouble Code
Low Circuit for Turbocharger/Supercharger Boost Control “B”
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to OBD-II equipped vehicles with a supercharger or turbocharger (Toyota, Hyundai, Ford Powerstroke, GMC Chevy Duramax, VW, Kia, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
Turbochargers and superchargers are air pumps that force air into the engine to increase power. Superchargers are driven by a belt connected to the engine crankshaft, while turbochargers are driven by the engine’s exhaust.
Many modern turbocharged vehicles use a variable geometry turbocharger (VGT), which has adjustable vanes located around the outside of the turbine. These vanes can be opened and closed to vary the amount of boost pressure, allowing the turbo to be controlled independently of engine speed. The position of the vanes is controlled by the powertrain control module (PCM) using an electronic control solenoid or motor. A dedicated position sensor is used to determine the turbo vane position.
In vehicles with a traditional fixed displacement turbocharger or supercharger, boost is controlled using a wastegate or bypass valve. This valve opens to relieve boost pressure, and the PCM monitors the system using a boost pressure sensor.
In the context of this trouble code, the “B” indicates a problem with a portion of the system circuit, rather than a specific symptom or component.
Code P004C is set when the PCM detects a low boost control solenoid signal, regardless of whether the engine uses a VGT turbo or a traditional turbocharger/supercharger.
Related turbo/supercharger engine trouble codes:
Code Severity & Symptoms
These codes have a moderate to severe severity. In some cases, problems with the turbocharger/supercharger can cause severe engine damage. It is advisable to address this code as soon as possible.
Symptoms of a P004C code may include:
Causes
Potential causes for this code to be set are:
Diagnostic and Repair Procedures
Begin by visually inspecting the turbo and turbo control system. Look for loose connections, damaged wiring, vacuum leaks, etc. Next, check for technical service bulletins (TSBs) related to the issue. If nothing is found, proceed with step-by-step diagnosis of the system.
The following is a generalized procedure, as testing for this code varies between vehicles. To accurately test the system, refer to the manufacturer’s diagnostic flow chart.
Check the system function by commanding the control solenoid to change position using a bidirectional scan tool. Raise the engine RPMs to about 1,200 RPMs and toggle the solenoid between on and off. While doing this, the engine RPMs should change and the sensor position PID on the scan tool should also change. If the RPMs fluctuate but the position/pressure PID does not change, suspect a problem with the sensor or its circuit. If the RPMs do not change at all, suspect a problem with the control solenoid, turbo/supercharger, or wiring.
