P0242

Turbocharger Boost Sensor B Circuit High

P0242 is a generic OBD-II powertrain diagnostic trouble code: Turbocharger Boost Sensor B Circuit High. 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
P0242
Group
Powertrain
System
Turbo/Boost
Severity
Warning (MIL on, possible limp mode)
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What P0242 means

P0242 is set when the ECM detects that the Boost Pressure Sensor B signal voltage has risen above its maximum calibrated threshold — typically above approximately 4.8–5.0 volts — at a time when high boost is not being commanded. This indicates the signal circuit is being pulled high, most commonly by an internal open circuit within the sensor element, or by the signal wire contacting a power source (short to voltage). When the sensor's internal resistance element fails open, the full 5-volt reference appears on the signal line without any voltage division, presenting as a false over-pressure reading.

The ECM reacts by treating the sensor as reporting extreme over-boost, immediately commanding a reduction in boost or entering limp mode — meaning P0242 paradoxically causes under-boost symptoms despite appearing to indicate high pressure. On twin-turbo platforms the code specifically targets the Sensor B bank. Unlike P0241 (shorted low), a high-circuit fault is more often caused by internal sensor element failure than by wiring damage, though a signal wire touching a 12-volt circuit can produce the same result.

After a turbo replacement or installation of an aftermarket boost controller, verify that sensor voltage at idle is approximately 0.5 V and not pegged near 5 V, which would indicate a sensor type mismatch or a wiring error where the signal and reference lines have been connected together.

Common causes

Most-frequently reported root causes when P0242 is logged.

  • 1
    Sensor B internal open circuit causing the full 5-volt reference to appear on the signal line.
  • 2
    Signal wire short to a 12-volt power source — e.g., contacting a switched supply in the harness bundle.
  • 3
    Swapped signal and reference wires at the sensor connector following recent service.
  • 4
    Corrosion inside the connector bridging the reference (5 V) and signal pins.
  • 5
    Incorrect or non-OEM replacement sensor with a different resistance characteristic that saturates the signal at low pressure.
  • 6
    Aftermarket boost controller or MAP signal enhancer intercepting and artificially raising the sensor output.
  • 7
    Damaged ECM sensor input circuit holding the pin high (rare, only after all other causes are excluded).

Symptoms drivers notice

Check Engine / MIL light illuminated.
Reduced boost output — ECM limits boost in response to the falsely high pressure reading.
Vehicle enters limp mode with noticeably poor acceleration, particularly at higher RPM.
Engine may run richer than normal as the ECM compensates for the perceived boost level.
Boost gauge reads low despite the ECM believing boost is excessively high.
On twin-turbo platforms, asymmetric power delivery with the Sensor B bank contributing less thrust.

How to diagnose P0242

A typical diagnostic flow when this code is present.

  1. 1
    Confirm P0242 with a scanner and note any companion codes — a simultaneous Sensor A high code (P0238) suggests a shared reference supply fault rather than an isolated Sensor B failure.
  2. 2
    With the key on and engine off (KOEO) and the sensor connected, measure the Sensor B signal voltage: a reading above 4.5-5 V at rest (no boost) confirms the high-circuit condition.
  3. 3
    Disconnect the Sensor B connector and re-measure the signal pin at the harness side: if the voltage drops to near 0 V, the sensor itself is the fault (internal open element); if the voltage remains high, a wiring short to power exists in the harness.
  4. 4
    Inspect the connector for corrosion bridges between the reference and signal pins, and verify no wiring was inadvertently swapped during recent service.
  5. 5
    Trace the signal wire for contact with any 12-volt switched circuits, particularly where the harness is routed near the fuse box or other power distribution points.
  6. 6
    Replace Sensor B if the sensor is confirmed faulty, then clear codes and verify signal voltage is approximately 0.5 V at idle before returning the vehicle to service.
  7. 7
    If a signal wire short to power is found, repair and protect the harness, confirm reference voltage is a clean 5.0 V, and validate with a full boost drive cycle.

Related powertrain codes

Frequently asked questions

Why does a high sensor reading cause low boost symptoms?

Because the ECM interprets Sensor B's exaggerated voltage as evidence of dangerously high boost pressure. As a protective measure it commands the wastegate to open more aggressively or limits fuelling, physically reducing actual boost — the engine is being deliberately de-rated in response to what the ECM believes is over-boost.

How can I quickly tell if P0242 is a wiring fault or a dead sensor?

Disconnect the sensor connector. With the key on, measure the signal wire at the harness side. If the voltage drops to near 0 V, the harness is clean and the sensor itself is pulling the line high (internal failure). If the voltage stays high with the sensor unplugged, a wiring short to power exists and the harness must be traced and repaired.

Could an aftermarket boost enhancer trigger P0242?

Yes. Piggyback boost signal interceptors that raise the sensor output voltage to fool the ECM into commanding more boost can overshoot the ECM's maximum expected voltage threshold and trigger P0242, especially if miscalibrated or used at unusually high boost levels beyond the sensor's design range.

Is P0242 more likely a sensor failure or a wiring failure?

Statistically, internal sensor failure (open element) is more common than a wiring short to voltage for P0242. The disconnect test described in diagnostic steps will confirm which is at fault within minutes and should be performed before chasing wiring faults.

Disabling P0242 in software

RaceTune can permanently disable P0242 — 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.

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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