"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. With this code the circuit is electrically intact, but the actuator does not follow the command plausibly: its actual position deviates too far or too long from the target, or its learned values do not fit. Boost is then too low or too high depending on where it sticks, often with reduced power.
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
- Turbine vanes or wastegate linkage sooted or sticking
- Electric actuator faulty
- Actuator learn value invalid, e.g. after low battery voltage at start
- Vacuum leak at a pneumatic actuator
- Actuator position sensor faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
- It affects emissions.
What each character of P004B means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 0: Subsystem depends on the code range
- 4B: Specific fault in that system
Frequently asked questions
Can I keep driving with code P004B?
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 P004B?
The most common causes are: turbine vanes or wastegate linkage sooted or sticking; electric actuator faulty; actuator learn value invalid, e.g. after low battery voltage at start.
Related codes
- P004ATurbocharger/Supercharger Boost Control B Circuit/Open
- P004CTurbocharger/Supercharger Boost Control B Circuit Low
- P004DTurbocharger/Supercharger Boost Control B Circuit High
- P003BTurbocharger/Supercharger Boost Control B Position Exceeded Learning Limit
- P0046Turbocharger/Supercharger Boost Control A Circuit Range/Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- saabwisonline.com: conditions for setting the dtc 193
- en.wikipedia.org: Twin turbo
- en.wikipedia.org: Variable geometry turbocharger
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.