Powertrain, generic · Fuel and air metering, auxiliary emission controls

P004C Turbocharger/Supercharger Boost Control B Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

"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.

Likely causes

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

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–900
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

boost control actuator bboost control actuator wiringengine control module

Related codes

P004AP004BP004DP005EP0047

References: law.cornell.edu, saabwisonline.com, en.wikipedia.org.