P0235
PowertrainTurbocharger/Supercharger Boost Sensor "A" Circuit
Most people notice this one as a sudden loss of go. The car pulls fine one minute, then it feels like someone's strapped a sandbag to it and the warning light is on. What's happened is the ECU has lost faith in the signal coming from the boost pressure sensor, the bit that tells it how hard the turbo or supercharger is pushing. Because the ECU can't trust that reading, it often drops the car into limp mode to protect the engine, so what you feel as 'no power' is usually the safety net kicking in, not the turbo itself being dead.
ⓘ Information only. This page provides general educational information about fault code P0235. 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 P0235 mean?
P0235 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Turbocharger/Supercharger Boost Sensor "A" Circuit.
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:
- • Warning light on plus a flat, gutless feeling under acceleration
- • Car drops into limp mode, holding revs low and refusing to build boost
- • Worse fuel economy, the ECU is fuelling blind so it tends to overdo it
- • If you've got a boost gauge fitted, readings that bounce around or sit too low
- • Hesitation when you put your foot down, especially pulling onto a motorway
- • Occasional whistle or rattle from the turbo area if there's a related leak
Possible causes
Causes commonly associated with P0235, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty boost pressure sensor sending a duff voltage. The sensor itself is the usual offender, they cook over the years sitting near a hot turbo
- 2. Corroded or loose connector at the sensor. These plugs sit in a filthy part of the engine bay and the pins corrode or back out
- 3. Chafed or broken wiring between the sensor and ECU, often where the loom rubs against a bracket or heat shield
- 4. Split or popped-off charge pipe letting boost escape before it reaches the sensor, which throws the reading right off
- 5. Oil or muck contaminating the sensor port, common on higher-mileage diesels that breathe oil through the intake
- 6. Wastegate actuator playing up so actual boost doesn't match what the ECU expects
- 7. Less often, genuine turbo wear (worn bearings, knackered vanes) that the sensor is correctly reporting
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 and watch the boost sensor while you blip the throttle. A healthy sensor tracks pressure smoothly. A flat line, a stuck value or wild spikes points you straight at the sensor or its wiring
- 2. Get under the bonnet and check the connector first. Unplug it, look for green corrosion or bent pins, and check the loom isn't rubbed through near the heat shield
- 3. Back-probe the sensor with a multimeter. Most run a 5V reference and signal that climbs with pressure, typically around 0.5V at atmosphere rising under boost. No 5V supply means a wiring or ECU-side fault, not the sensor
- 4. Pressure-test or eyeball the charge pipes and intercooler hoses for splits and loose clamps. A blown boost pipe mimics a sensor fault and is a far cheaper fix
- 5. Check continuity back to the ECU pins if the supply and signal look wrong at the sensor. You're hunting an open or a short in the harness
- 6. Only if sensor, wiring and pipework all check out should you start suspecting the wastegate or the turbo itself
Common questions about P0235
What's the worst that happens if I just keep driving it like this? +
Two things. If the car's in limp mode it's miserable to drive and slow to pull out of junctions, which is a hazard on faster roads more than an engine risk. The bigger worry is if the underlying cause is a boost leak or a wastegate fault rather than just the sensor, because then the engine could be over- or under-boosting without the ECU seeing it properly. A short drive to the garage is fine. Living with it for weeks is asking for trouble, especially on a turbo diesel.
How quickly do I actually need to sort this? +
Days, not weeks. It's not a roadside-recovery emergency in most cases, the car will limp home. But because boost faults can run into expensive turbo damage if a real mechanical problem is being masked, get it looked at while it's cheap to diagnose. If it keeps dropping into limp mode or you smell anything hot or oily, move it up the list.
Is it the sensor itself or just the wiring and plug? +
Honestly it's a coin toss until you've checked. The sensor is a common failure, they're cheap enough at roughly £20 to £60 and easy to fit. But a corroded connector or a chafed wire near the heat shield causes the exact same code and a new sensor won't fix that. Always check the plug and the supply voltage before throwing a part at it. People replace the sensor, clear the code, and it's back the next day because the real fault was a green, crusty connector all along.
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 →