P0240
PowertrainTurbocharger/Supercharger Boost Sensor "B" Circuit Range/Performance
The ECU watches the second boost pressure sensor (sensor B) and cross-checks what it reports against the MAP sensor and what it expects the turbo to be producing. When that signal drifts outside its normal window, sticks at a flat value, or stops tracking the other sensors properly, the ECU flags P0240. For you as the driver, that usually shows up as the warning light and a car that suddenly feels gutless, because once the ECU can't trust the boost reading it pulls power back to protect the engine.
ⓘ Information only. This page provides general educational information about fault code P0240. 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 P0240 mean?
P0240 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Turbocharger/Supercharger Boost Sensor "B" Circuit Range/Performance.
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 on, sometimes with a clear loss of poke at the same time
- • Flat acceleration, particularly when you put your foot down on a motorway slip road
- • Car drops into limp mode, capping the revs and often killing the air con
- • Worse fuel economy, the engine ends up working harder for the same speed
- • Sluggish throttle response higher up the rev range where the turbo should be pulling hard
- • On some engines a rough or hunting idle when the sensor reading is all over the place
Possible causes
Causes commonly associated with P0240, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The boost sensor B itself worn out or contaminated, common as the sensor ages and oil mist coats the element. This is the first suspect on higher-mileage cars
- 2. Wiring and connector trouble in the sensor circuit, corroded pins, a chafed loom, or a connector that has worked loose with engine vibration. Heat and damp in the engine bay take their toll here
- 3. Split, kinked or perished vacuum and pressure hoses feeding the sensor, so it never sees the real pressure
- 4. Blocked or gunged-up sensor port on the manifold or turbo, choking off the reading
- 5. An intake or boost leak letting pressure escape before the sensor sees it, which throws the cross-check out
- 6. Wastegate solenoid or mechanical wastegate playing up, so actual boost doesn't match what the ECU commands
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 the live data and the freeze frame, then compare sensor B against the MAP and barometric reading with the engine off and key on. They should all sit close together at rest. If sensor B is way out on its own, you've found your problem area
- 2. Back-probe the sensor connector and check the output voltage with a multimeter, it should sweep cleanly through roughly 0.5 to 4.5 volts. A flat or jumping signal points at the sensor or its wiring
- 3. Wiggle-test the loom and connector while watching live data, a reading that flickers as you move the harness tells you it's a wiring fault, not the sensor
- 4. Inspect every vacuum and pressure hose to the sensor for splits, kinks and oil soaking, and check the sensor port isn't clogged
- 5. Check the wastegate solenoid is getting vacuum and actuating, and confirm actual boost is tracking commanded boost
- 6. Clear the code and drive it under load, on and off boost, to see whether it comes straight back
Common questions about P0240
How do I know if it's the sensor or just the wiring behind it? +
The cheapest answer first: wiggle the connector and loom while you watch the sensor's live voltage. If the reading jumps or drops out as you move the harness, you're chasing a wiring or connector fault, and that's a clean-up or repair job rather than a new part. If the signal is steady but reads obviously wrong compared to the MAP sensor at rest, the sensor is the more likely culprit. Plenty of these turn out to be a corroded pin or a connector full of damp, especially on cars that live outside, so don't condemn the sensor before you've checked the plug.
How long is this in the garage for? +
If it lands on a sensor or a hose, you're looking at a quick job, maybe half an hour to an hour once the diagnosis is done, because most boost sensors are easy to get at on the manifold or pipework. The diagnosis itself can eat more time than the fix, particularly if it's an intermittent wiring fault that only shows under load. If it ends up being a wastegate or turbo issue dressed up as a sensor code, that's a different conversation and a longer stay.
Is a cheap aftermarket boost sensor any good, or should I stick with the genuine part? +
A decent branded aftermarket sensor from a known supplier like Bosch or VDO is fine for daily driving and a fraction of dealer money. Where people get caught out is the no-name eBay sensors, which often read slightly off and set the same code straight back, leaving you to buy it twice. On a turbo diesel that's fussy about boost mapping, I'd lean towards OEM or a top-tier aftermarket brand rather than the cheapest one in the listing. Spend the extra few pounds and buy it once.
Can I keep driving with this showing? +
A short hop home is usually fine, but I wouldn't make a habit of it. Many cars drop into limp mode with this code, so you'll be crawling along with no real power, and driving hard while the ECU can't trust the boost reading risks overfuelling and stressing the turbo and exhaust. Get it looked at sooner rather than later, because a £100 sensor left ignored can turn into a much bigger bill if the underlying boost control is actually at fault.
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 →