"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 control circuit is open: with the driver switched off no load can be measured, or the measured state does not match the command. 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.
Medium severity. Can I keep driving?Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
Symptoms
- Check engine light on
- Loss of power or reduced power mode
- Weak acceleration, noticeable turbo lag
- Overboost possible in rare cases
Most likely causes
- Connector at the actuator corroded or loose
- Wire broken or chafed through
- Solenoid coil in the actuator burnt out
- Fuse or ignition supply faulty
- Engine control module or separate actuator control unit faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P004A means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 0: Subsystem depends on the code range
- 4A: Specific fault in that system
Frequently asked questions
Can I keep driving with code P004A?
Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
What causes code P004A?
The most common causes are: connector at the actuator corroded or loose; wire broken or chafed through; solenoid coil in the actuator burnt out.
Related codes
- P004BTurbocharger/Supercharger Boost Control B Circuit Range/Performance
- P004CTurbocharger/Supercharger Boost Control B Circuit Low
- P004DTurbocharger/Supercharger Boost Control B Circuit High
- P004FTurbocharger/Supercharger Boost Control B Circuit Intermittent/Erratic
- P005ETurbocharger/Supercharger Boost Control B Supply Voltage Circuit Low
- P0045Turbocharger/Supercharger Boost Control A Circuit/Open
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.