P0238

Turbocharger Boost Sensor A Circuit High

P0238 is a generic OBD-II powertrain diagnostic trouble code: Turbocharger Boost Sensor A 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
P0238
Group
Powertrain
System
Turbo/Boost
Severity
Critical (limp mode / no-start)
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What P0238 means

P0238 is triggered when the ECM detects that the boost pressure sensor 'A' output voltage has exceeded the upper allowable threshold — typically above approximately 4.5 V on a 5 V reference circuit. The sensor operates as a variable resistor: as intake pressure rises, its resistance falls, allowing more of the 5 V reference to pass back to the ECM as a proportional signal. A signal that pegs near or above 4.5 V indicates the circuit is shorted to the reference or battery voltage, the sensor is internally failed in a shorted state, or — less commonly — the turbocharger is genuinely producing extreme over-boost beyond the sensor's calibrated range due to a failed wastegate or boost-control valve.

When P0238 is stored the ECM disregards the boost sensor reading and substitutes a conservative fixed value, simultaneously limiting boost, capping engine rpm, and retarding dynamic fuel timing. This 'limp home' mode is intended to protect the turbocharger and engine from sustained over-boost damage, and on most vehicles it is very noticeable as a drastic reduction in acceleration. The MIL illuminates in the same drive cycle the fault is detected.

P0238 is the mirror image of P0237 (circuit low): where P0237 indicates a short to ground or open reference, P0238 indicates a short to the supply voltage or an open ground. Both fault types result in the ECM losing accurate boost feedback and triggering protective limp mode. Accurate wiring diagnosis — checking reference voltage, signal voltage, and ground integrity — is essential before replacing the sensor.

Common causes

Most-frequently reported root causes when P0238 is logged.

  • 1
    Boost sensor signal wire shorted to the 5 V reference wire or to battery voltage, driving the signal above the ECM high-voltage threshold.
  • 2
    Open sensor ground wire forcing the signal to float toward the reference voltage.
  • 3
    Internally failed boost sensor with a shorted sensing element locking the output near maximum voltage.
  • 4
    Damaged wiring harness near the turbocharger — heat or chafing causing the signal wire to contact the reference or battery supply.
  • 5
    Failed wastegate actuator or boost-control solenoid causing genuine mechanical over-boost that exceeds the sensor's calibrated upper range.
  • 6
    Corroded or loose connector pins increasing circuit resistance in a way that shifts the signal reading toward the upper limit.
  • 7
    ECM analog-to-digital input circuit failure misinterpreting a normal signal as above-threshold (rare; diagnose after excluding external faults).

Symptoms drivers notice

Check Engine Light illuminated, often with an 'Engine Power Limited' message on the instrument cluster.
Severe reduction in engine power — ECM limits boost and rpm as a protective measure.
Erratic or pegged boost pressure gauge reading (if fitted) showing maximum pressure regardless of engine load.
Increased fuel consumption as the ECM runs on a fixed boost estimate rather than actual measured values.
Possible turbocharger noise or over-boost event if the root cause is a failed wastegate rather than a sensor or wiring fault.

How to diagnose P0238

A typical diagnostic flow when this code is present.

  1. 1
    Scan for all codes and capture freeze frame data; companion codes such as P0234 (over-boost) alongside P0238 suggest a possible mechanical wastegate fault rather than a pure sensor/wiring fault.
  2. 2
    With the ignition on, engine off, measure the boost sensor signal voltage — a reading above 4.5 V confirms the fault is present statically; normal is approximately 1.5–2.0 V at atmospheric pressure.
  3. 3
    Check the 5 V reference voltage and the sensor ground at the connector; if ground is open (no continuity to chassis), locate and repair the open ground before replacing the sensor.
  4. 4
    Inspect the wiring harness near the turbocharger for heat damage, chafing against the exhaust manifold, or contact between the signal and reference wires.
  5. 5
    Disconnect the sensor: if the signal voltage drops to near zero immediately, the sensor is internally shorted and requires replacement.
  6. 6
    If the signal normalises with a known-good substitute sensor, replace the original sensor and retest.
  7. 7
    If electrical tests are all normal, perform an actual boost pressure test with a mechanical gauge to rule out genuine over-boost caused by a failed wastegate or boost-control valve.

Related powertrain codes

Frequently asked questions

What voltage triggers P0238?

The exact threshold varies by manufacturer, but the ECM generally flags P0238 when the boost sensor signal exceeds approximately 4.5 V on a 5 V reference circuit. At idle with no boost, the sensor should return roughly 1.5–2.0 V; under full boost it typically reaches 3.5–4.0 V. A reading above 4.5 V represents a hardware limit that the sensor should never reach during normal operation.

Could a stuck-open wastegate cause P0238?

A stuck-closed wastegate (or a failed boost-control solenoid unable to open the wastegate) causes genuine over-boost. If the actual intake pressure exceeds the sensor's upper calibrated range, the sensor output can peg at maximum voltage, setting P0238. Always verify actual boost pressure with a mechanical gauge when wiring and sensor tests pass normally.

Is P0238 the opposite of P0237?

Yes, electrically. P0237 fires when the signal is below the minimum (~0.5 V), caused by a short to ground or open reference supply. P0238 fires when the signal is above the maximum (~4.5 V), caused by a short to the supply voltage or an open sensor ground. Both result in the ECM substituting a fixed boost value and activating limp mode.

Will the engine be damaged if I keep driving with P0238?

The ECM's limp mode limits boost to protect the engine in the short term, but if the underlying cause is genuine over-boost from a failed wastegate, continued driving risks catastrophic turbocharger failure, blown intake intercooler hoses, or internal engine damage from excessive cylinder pressure. The vehicle should be diagnosed promptly.

Disabling P0238 in software

RaceTune can permanently disable P0238 — 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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