P0238
PowertrainTurbocharger/Supercharger Boost Sensor "A" Circuit High
The turbocharger boost pressure sensor 'A' circuit is reading too high. Either the sensor itself has failed in a high-voltage state, or there's genuinely too much boost pressure in the system, or the wiring has shorted to a higher voltage source.
ⓘ Information only. This page provides general educational information about fault code P0238. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0238 mean?
P0238 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Turbocharger/Supercharger Boost Sensor "A" Circuit High.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light
- • Possibly limp mode, particularly if the ECU thinks boost is dangerously high
- • Loss of power, sometimes only at higher revs
- • Inconsistent acceleration, surge followed by sudden flatness
- • On petrol turbos: occasionally a fluttering or 'chuff' sound on lift-off
- • On diesels with overboost protection: smoky black exhaust under load before limp mode kicks in
Possible causes
Causes commonly associated with P0238, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed boost sensor reading high (most common, sensors fatigue and sometimes fail in this state)
- 2. Wiring shorted to a higher voltage source at any point in the run
- 3. Connector contaminated or with backed-out pins, conductive contamination bridging signal and supply pins
- 4. Genuinely high boost from a stuck-closed wastegate or stuck variable-vane mechanism
- 5. On remapped cars: the tune is pushing boost beyond what the sensor expects
- 6. Less commonly: ECU 5V reference fault affecting all sensors on that rail
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read live data on commanded vs actual boost throughout the rev range; if both show similar high values, you have genuine overboost; if commanded is normal but actual reads abnormally high, the sensor or wiring is the issue
- 2. Inspect sensor for oil contamination from a leaking turbo seal
- 3. Check connector and wiring for damage, particularly looking for chafing where the loom touches conductive surfaces
- 4. Test sensor electrically against spec at known reference pressures using a hand vacuum/pressure pump
- 5. If wiring and sensor are healthy and actual boost is genuinely high, look at the wastegate or variable-vane control
Common questions about P0238
Same as P0234? +
Closely related. P0234 is specifically system overboost (the turbo is producing more boost than commanded, and the ECU has detected genuinely excess pressure). P0238 is the sensor reading high (could be sensor fault or genuine overboost). The diagnostic approach starts by determining which it is via live data: if requested and actual both look reasonable but the sensor reads high, the sensor is at fault; if actual is genuinely high, you have a turbo control problem.
Could a recent remap have caused this? +
Yes, especially on aggressive tunes. If the new tune commands more boost than the stock sensor's range, the sensor reads at maximum and P0238 sets. Some tuners specify uprated sensors as part of their map; if your tuner didn't, that's a possible cause. Have the tune reviewed.
Is overboost dangerous? +
Yes if sustained. Genuine overboost can blow intercooler hoses, damage turbo bearings, and push detonation that damages pistons. The ECU dropping into limp mode is the protection; don't override it. Get diagnosed properly.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →