P278A

Turbocharger/Supercharger Boost Control A Circuit Low

P278A is a generic OBD-II powertrain diagnostic trouble code: Turbocharger/Supercharger Boost Control A Circuit Low. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P278A
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
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What P278A means

P278A is set when the PCM detects a lower-than-expected voltage or current on the turbocharger or supercharger boost control circuit A. This circuit typically drives an electronic wastegate actuator, variable geometry turbocharger (VGT) vane position solenoid, or boost pressure control solenoid, depending on the engine design. The 'A' designation refers to the first or primary boost control actuator on the system.

When the control circuit is in a low state, the actuator may not receive sufficient drive signal to modulate boost pressure correctly. The result is often uncontrolled or fixed wastegate/vane position, causing boost to default to the mechanical minimum or maximum depending on the actuator design. This can manifest as reduced power or, in some cases, an overboost condition with associated safety limiters activating.

Common root causes are a short to ground in the solenoid wiring, a failed solenoid with a shorted coil, or a degraded PCM driver circuit. Because boost directly affects engine load calculations, a fault here can cascade into multiple related codes including MAP sensor faults and throttle response issues.

Common causes

Most-frequently reported root causes when P278A is logged.

  • 1
    Short to ground in the boost control solenoid A wiring harness.
  • 2
    Shorted solenoid coil winding in the wastegate or VGT actuator.
  • 3
    Corroded or damaged connector at the boost control solenoid.
  • 4
    Failed PCM driver circuit outputting low voltage for the boost control channel.
  • 5
    Chafed wiring contacting engine ground or chassis ground nearby.
  • 6
    Moisture intrusion into the solenoid connector causing a resistive short.
  • 7
    Aftermarket boost controller wiring interference with the OEM circuit.

Symptoms drivers notice

MIL illuminated.
Reduced engine power or noticeable loss of boost pressure.
Possible limp mode with boost pressure capped at a safe default.
Turbocharger may make unusual noise if vanes are stuck in one position.
Potential overboost condition if the actuator defaults to a closed wastegate position.
Additional boost or MAP sensor fault codes may accompany P278A.

How to diagnose P278A

A typical diagnostic flow when this code is present.

  1. 1
    Scan for all DTCs and record freeze frame; note any companion boost pressure or MAP sensor codes.
  2. 2
    Inspect the boost control solenoid A wiring and connector for short circuits, chafing, or corrosion.
  3. 3
    With the solenoid disconnected, measure solenoid coil resistance and compare to specification; a very low or zero reading indicates an internal short.
  4. 4
    With the circuit disconnected at the PCM, check for continuity between the solenoid control wire and ground to identify a short-to-ground in the harness.
  5. 5
    Command the solenoid on and off using scan tool output controls and verify circuit behavior with a lab scope or voltmeter.
  6. 6
    Inspect the physical turbocharger actuator for mechanical binding that could cause electrical overload.
  7. 7
    Repair wiring faults or replace the solenoid as indicated; retest and verify boost pressure response.

Vehicles where we've handled P278A

Platforms in our catalogue with confirmed P278A coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Will P278A put the car in limp mode?

Often yes. Most calibrations protect the engine by limiting boost when the control circuit is faulted, which typically results in reduced power output.

Could the turbocharger itself cause this code?

The turbocharger mechanical components do not directly cause electrical circuit codes, but a seized VGT actuator can overload the solenoid driver, contributing to a circuit fault.

What is boost control circuit A?

In systems with multiple turbochargers or staged boost, circuit A refers to the primary actuator. On single-turbo engines it is simply the one wastegate or VGT solenoid.

Is P278A dangerous to ignore?

Ignoring it risks operating with uncontrolled boost. Depending on actuator default position, this could result in chronic underboost (annoying but safe) or overboost (potentially damaging to the engine).

Disabling P278A in software

RaceTune can permanently disable P278A — 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.

ECUs with a P278A disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C50 verified 1 software version
  • Bosch EDC17CP09 verified 1 software version

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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