P0239
Turbocharger Boost Sensor B Circuit MalfunctionP0239 is a generic OBD-II powertrain diagnostic trouble code: Turbocharger Boost Sensor B Circuit Malfunction. 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.
What P0239 means
P0239 is set when the Engine Control Module (ECM) detects an abnormal or implausible signal from the Boost Pressure Sensor B circuit. On vehicles equipped with a single turbocharger, Sensor B is typically a secondary pressure sensor mounted after the intercooler or on a second intake runner. On twin-turbocharger platforms (such as sequential or parallel twin-scroll systems), Sensor B monitors the second turbocharger's outlet independently of Sensor A, allowing the ECM to compare both readings for rationality.
The ECM supplies a 5-volt reference to the sensor and monitors the return voltage, which should scale linearly with boost pressure. A generic circuit malfunction code means the ECM detected a signal that is outside the normal operating range — either too high, too low, or erratic — without the fault being specifically classified as purely high or low. The MIL illuminates and the ECM may substitute a default boost value, reducing power output as a precaution.
Common causes include a failed sensor with an internally degraded element, corrosion or water intrusion at the sensor harness connector, a chafed or shorted signal wire, or a cracked pressure port hose that prevents the sensor from seeing true manifold pressure. In rare cases a faulty ECM output driver for the 5-volt reference supply can affect all pressure sensors simultaneously.
Common causes
Most-frequently reported root causes when P0239 is logged.
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1
Faulty Boost Pressure Sensor B (internal element failure or contamination).
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2
Corroded, loose, or water-damaged harness connector at Sensor B.
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3
Chafed, shorted, or open-circuit signal wire in the Sensor B harness.
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4
Cracked or disconnected pressure port hose between intake manifold/pipe and Sensor B.
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5
Loss of 5-volt reference supply to the sensor (shared rail fault affecting multiple sensors).
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6
Boost leak upstream of Sensor B causing abnormal pressure readings.
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7
Faulty ECM (rare — only after all wiring and sensor checks pass).
Symptoms drivers notice
How to diagnose P0239
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scanner, confirm P0239 and note any companion codes (P023x, P024x, MAP-related).
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2
Visually inspect the Boost Sensor B connector and harness for corrosion, water ingress, backed-out pins, or heat damage.
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3
With the key on and engine off (KOEO), back-probe the sensor signal pin and verify the 5-volt reference is present; an absent reference indicates a wiring or ECM supply fault.
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4
Check the sensor's pressure port hose for cracks, kinks, or disconnection that would prevent accurate pressure sensing.
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5
With the engine running, monitor Sensor B voltage live; it should rise smoothly with throttle from approximately 0.5 V at idle to near 4.5 V at full boost — a flat or erratic reading confirms sensor failure.
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6
Inspect the wiring harness back to the ECM for chafing against the engine block, exhaust, or turbo heat shield.
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7
Replace Boost Pressure Sensor B if voltage is abnormal after confirming good reference and wiring, then clear codes and retest under load.
Related powertrain codes
- P003A — Turbocharger/Supercharger Boost Control A Position Exceeded Learning Limit
- P003B — Turbocharger/Supercharger Boost Control B Position Exceeded Learning Limit
- P0045 — Turbocharger/Supercharger Boost Control A Circuit/Open
- P0046 — Turbocharger/Supercharger Boost Control A Circuit Range/Performance
- P0047 — Turbocharger/Supercharger Boost Control A Circuit Low
- P0048 — Turbocharger/Supercharger Boost Control A Circuit High
Frequently asked questions
What is Boost Sensor B and how does it differ from Sensor A?
Sensor A is the primary boost pressure sensor, typically on the compressor outlet or intake manifold. Sensor B is a secondary sensor — on twin-turbo engines it monitors the second turbo's outlet independently; on single-turbo setups it may be located post-intercooler to measure charge air pressure separately from the pre-intercooler reading.
Can I drive with P0239 stored?
Short distances are possible but not advisable. The ECM will likely limit boost and may run a default fuelling map, reducing performance and potentially allowing undetected over-boost or under-boost conditions that could damage the turbocharger or engine over time.
Will clearing the code fix the problem?
No. Clearing the code without diagnosing and repairing the root cause will result in the code returning once the fault condition is detected again, typically within the next drive cycle under boost.
Could a boost leak trigger P0239 even if the sensor itself is fine?
Yes. A boost leak downstream of the turbo but upstream of Sensor B will cause the sensor to see lower pressure than the ECM commands, producing a rationality mismatch that can register as a circuit malfunction depending on the manufacturer's detection threshold.
Disabling P0239 in software
RaceTune can permanently disable P0239 — 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.
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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