P004C: Turbocharger/Supercharger Boost Control B Circuit Low

What does P004C mean?

"Boost control B" is the electrically driven actuator the engine control module uses to set boost pressure: depending on the engine a pulse-width modulated solenoid or vacuum converter at the wastegate, the control valve of variable turbine vanes, or an electric actuator with position feedback. B is the second actuator, for example on the high-pressure turbocharger of a two-stage system or on the second turbocharger; it does not denote cylinder bank 2. This code is set when the voltage at the control output is too low, typically because of a short to ground; some manufacturers also report a missing supply this way. Without control the actuator usually falls back to its rest position, which on most engines means low boost; depending on the design too much boost is also possible.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Loss of power or reduced power mode
• Weak acceleration, noticeable turbo lag
• Overboost possible in rare cases

Common causes

• High Harness chafed, short to ground
• Medium Actuator coil shorted
• Medium Water in the connector
• Low Output driver in the engine control module faulty

Likelihood is a general estimate across vehicles; your make and model may differ.