P004B

Turbocharger/Supercharger Boost Control B Circuit Range/Performance

P004B is a generic OBD-II powertrain diagnostic trouble code: Turbocharger/Supercharger Boost Control B Circuit Range/Performance. It is logged by the engine control unit when the turbo/boost monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P004B
Group
Powertrain
System
Turbo/Boost
Severity
Warning (MIL on, possible limp mode)
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What P004B means

P004B is stored when the powertrain control module detects an open circuit or a general circuit fault in the charge air cooler (intercooler) bypass control circuit — on some calibrations this code is alternatively defined as Turbocharger/Supercharger Boost Control "B" Circuit Range/Performance. The charge air cooler bypass valve diverts compressed intake air around the intercooler core during specific operating conditions (such as cold ambient starts or rapid throttle transients) to prevent charge-air temperature from dropping too low or to improve throttle response. The PCM commands the bypass actuator via a solenoid or motor driver and monitors feedback to confirm the valve has reached the target position; when the circuit is open — no current flows because of a broken wire, failed solenoid, or disconnected connector — the PCM cannot drive the valve, logs P004B, and illuminates the MIL. Depending on the vehicle, limp-home mode may be engaged to protect the turbocharger from uncontrolled boost. Performance, fuel economy, and cold-start emissions are all affected when the bypass valve cannot be controlled precisely.

Common causes

Most-frequently reported root causes when P004B is logged.

  • 1
    Open or broken wire in the bypass actuator control circuit
  • 2
    Failed bypass control solenoid or actuator motor (open winding)
  • 3
    Disconnected, corroded, or damaged harness connector at the bypass valve
  • 4
    Blown fuse or fusible link in the actuator supply circuit
  • 5
    Faulty PCM control driver unable to energise the solenoid circuit
  • 6
    Vacuum line disconnected or cracked (on vacuum-operated bypass valves)
  • 7
    Mechanical seizure of the bypass valve preventing feedback confirmation (on position-sensored systems)

Symptoms drivers notice

Malfunction indicator lamp illuminated
Reduced turbo boost and noticeable loss of engine power or acceleration
Possible activation of limp-home mode limiting RPM or boost pressure
Rough or hesitant throttle response, particularly during transient acceleration
Potentially sluggish cold-start performance if the valve is stuck in bypass position
Possible companion boost pressure or turbocharger performance codes

How to diagnose P004B

A typical diagnostic flow when this code is present.

  1. 1
    Connect an OBD-II scan tool and read all codes; note any companion turbocharger, boost pressure, or MAF sensor codes that help define system context
  2. 2
    Visually inspect the bypass valve actuator connector and wiring harness for corrosion, broken pins, or damaged insulation
  3. 3
    Check the actuator supply fuse and replace if blown; investigate the cause before declaring repair complete
  4. 4
    Disconnect the actuator connector and measure solenoid winding resistance; an open reading (OL) indicates a failed solenoid requiring replacement
  5. 5
    With ignition on and actuator disconnected, verify supply voltage is present at the connector power terminal
  6. 6
    Measure continuity of the control wire from actuator connector to the PCM output pin; any open indicates harness damage between those points
  7. 7
    Use a bidirectional scan tool to command the bypass valve open and closed; verify actuator movement and any position-sensor PID response before condemning the PCM

Related powertrain codes

Frequently asked questions

What does the charge air cooler bypass valve actually do?

The bypass valve routes compressed, hot charge air around the intercooler core when bypassing the cooler is beneficial — for example, during cold starts to prevent intake air from being chilled below the dew point, or during rapid throttle stabs to reduce turbo lag. The PCM modulates the valve position for optimal charge temperature and response.

Will P004B cause my car to go into limp mode?

It depends on the vehicle calibration. Many turbocharged vehicles will enter a reduced-boost or reduced-RPM limp mode when the boost control circuit has an open fault, to protect the engine from uncontrolled pressure. If you experience sudden power loss alongside the MIL, assume limp mode and schedule prompt repairs.

Is P004B the same as P004B on all vehicle makes?

The SAE generic definition is Turbocharger/Supercharger Boost Control "B" Circuit Range/Performance, but several manufacturers use P004B to indicate an open in the charge air cooler bypass control circuit specifically. Always check a manufacturer-specific repair manual or ALLDATA to confirm the exact definition for the vehicle being diagnosed.

Can a vacuum leak trigger P004B on vacuum-actuated bypass valves?

Yes. On systems where the bypass valve is vacuum-operated, a cracked or disconnected vacuum hose will prevent the valve from responding to PCM commands. The PCM may detect the lack of valve movement via a position sensor and log P004B. Inspect all vacuum lines to the bypass valve actuator as part of initial diagnosis.

Disabling P004B in software

RaceTune can permanently disable P004B — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECU families we can disable P004B on

We hold the DaVinci A2L disable definitions for these families, so the exact P004B path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.

  • Bosch EDC17C50 verified
  • Bosch MD1CP002 verified
  • Bosch MD1CS001 verified

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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