Why is P0240 different from P0241 and P0242?

Because it is a plausibility code rather than a circuit code. P0241 says the sensor "B" signal is stuck low and P0242 says it is stuck high — both are electrical faults with fairly short suspect lists. P0240 says the signal is electrically normal but does not make sense for the conditions. That opens up a much wider search, because either the sensor is reporting the wrong number or the sensor is correct and boost pressure genuinely is not where the module wanted it. Those are completely different repairs, which is why the first real test on P0240 is comparing the sensor against a mechanical gauge rather than probing wires.

Could P0240 be a boost leak rather than a bad sensor?

Very often, yes, and that is the most useful thing to know about this code. A healthy sensor accurately reporting that boost never reached target sets the same code as a drifted sensor reporting a fictional number. Cracked charge pipes, loose clamps, failed silicone couplers, split intercooler end tanks, and a wastegate stuck open all produce a real shortfall the module reads as implausible. Verify the pressure independently with a mechanical gauge before you buy a sensor, then pressure-test the charge air system if the gauge and the sensor agree.

My car is tuned. Can the tune cause P0240?

It can, and this is a genuinely common scenario. Boost plausibility limits live in the calibration, and if boost targets have been raised beyond what the stock tables anticipate, the module can flag perfectly healthy hardware as out of range. The same applies to hardware changes the calibration was never told about, such as a larger turbo or a different intercooler. On a modified vehicle, rule out the calibration before condemning parts — and if the tune is the cause, the fix is a corrected tune, not a new sensor.

Which sensor is boost sensor "B"?

It depends on the engine, so look it up before buying anything. On twin-turbo engines "B" is usually the sensor for the second turbo or the second bank. On sequential systems it often manages the handover between the small and large turbos. On many single-turbo diesels the two sensors sit on either side of the charge air cooler and "B" is the post-cooler one — which is why a P0240 on those engines can be the module reporting that the cooler's pressure drop is implausible, making it a cooler or piping problem rather than a sensor problem.

OBD-II trouble code

P0240: Turbocharger/Supercharger Boost Sensor "B" Circuit Range/Performance

The second boost pressure sensor is reporting a signal that is electrically valid but does not agree with what the module expects. Unlike the circuit codes, this one frequently points at a real air-side problem — a boost leak, a stuck wastegate, or a charge air cooler fault — rather than at the sensor.

Medium severityPowertrainFuel & AirDrivable short-term

Quick facts

System: Powertrain
Category: Fuel & Air
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $110 – $1,400
DIY difficulty: Intermediate DIY

What does P0240 mean?

P0240 is a plausibility code, and understanding that distinction is what keeps this diagnosis from turning into an expensive guess. The module is not saying the sensor "B" circuit is shorted or open — that would be P0241 or P0242. It is saying the signal arriving from that sensor is within its normal electrical range but does not match what the module expects given engine speed, load, throttle position, barometric pressure, and what the other boost sensor is reporting. The circuit works. The number is just wrong, or it is right and something else is wrong.

That is the crucial fork in the road. A range and performance code on a pressure sensor has two entirely different explanations, and they lead to completely different repairs. Either the sensor is lying — drifted, contaminated, slow to respond, or fed by a blocked reference passage — or the sensor is telling the truth and the boost pressure genuinely is not where the module wanted it. The second case is more common than people expect, because a healthy sensor reporting a real shortfall in boost looks identical to a bad sensor until you verify the pressure independently.

So the single most valuable test on this code is comparing the sensor's live reading against a mechanical boost gauge teed into the intake under load. If the two agree, the sensor is fine and you are looking for a genuine air-side fault: a cracked or disconnected charge pipe, a split intercooler end tank, a loose clamp, a leaking intake gasket, a wastegate stuck open, or a turbo that is no longer making its target. If they disagree, the sensor or its circuit is at fault and the air side is a red herring.

Because this is sensor "B", the first thing to establish is which physical sensor that is on your engine. A second boost sensor only exists where the engine needs one — twin-turbo layouts with one sensor per turbo or per bank, sequential systems that manage a handover between a small and a large turbo, and a very large number of modern turbodiesels that place one sensor before the charge air cooler and another after it so the module can confirm the cooler is working. On that last arrangement, a P0240 can be the module noticing that the pressure drop across the cooler is implausible, which is a charge air cooler or piping complaint rather than a sensor complaint at all. Some sources and some scan tools label this code as a MAP sensor "B" turbo-system range and performance fault, which is the same idea described from the manifold-pressure side.

One more cause worth naming plainly: aftermarket tuning. If boost targets have been raised beyond what the stock control tables anticipate, a stock-calibration module can flag the resulting pressure as implausible even though every component is healthy. On a modified vehicle this code is often the calibration objecting rather than a part failing, and the fix is the tune, not a sensor.

For the driver, the consequences vary with how much the strategy depends on that sensor. Where it feeds cooler-efficiency monitoring, you may see nothing but the light. Where it participates in boost control, the module will normally take the cautious path and cap boost or drop into reduced power, because it will not manage boost aggressively on a signal it does not trust.

Common causes

Genuine boost leak — cracked or disconnected charge pipe, split intercooler end tank, loose clamp, or failed coupler
Drifted, contaminated, or slow-responding boost pressure sensor "B"
Blocked, oil-fouled, pinched, or misrouted pressure reference hose or intake port
Charge air cooler leak or restriction producing an implausible pressure differential between the two sensors
Wastegate stuck open or a leaking wastegate actuator diaphragm, so real boost never reaches target
Failing or worn turbocharger not achieving its commanded boost
Restricted air filter or collapsed intake ducting
Exhaust leak upstream of the turbine reducing available exhaust energy
Wrong-range or incorrect replacement sensor fitted in the "B" position
Aftermarket tune or boost modification pushing pressure outside stock plausibility limits
High-resistance ground or degraded reference voltage skewing an otherwise good sensor's output

Symptoms

Check engine light with P0240 stored
Reduced power, sluggish acceleration, or a boost cap
Reduced-power or limp mode on platforms that lean on this sensor for boost control
Boost target and actual boost diverging in live data
Audible hissing or whistling if there is a real charge-air leak
Poor fuel economy
Black smoke on diesel applications
Hesitation or an uneven handover on sequential-turbo systems
Sometimes no driveability complaint at all where the sensor is used only for monitoring

Diagnostic steps

1.Record all stored codes first. P0241 or P0242 alongside P0240 means the circuit itself is also faulted, which changes the priority — fix the circuit fault first.
2.Identify which physical sensor your engine calls boost sensor "B" using service data for that specific engine, since "B" is not a fixed location.
3.With the key on and the engine off, compare sensor "A" and sensor "B" in live data. Both should report roughly barometric pressure. A meaningful disagreement at rest condemns a sensor or its circuit without any driving.
4.Tee a mechanical boost gauge into the intake and compare it against the sensor reading on a controlled test drive under load. This is the test that separates a lying sensor from a real boost shortfall.
5.If the gauge agrees with the sensor, stop chasing electronics and pressure-test the intake and charge air system for leaks — clamps, couplers, intercooler end tanks, and the piping seams.
6.Inspect the charge air cooler and its piping for leaks, impact damage, and internal restriction, especially where the two sensors straddle the cooler.
7.Check the pressure reference hose or intake port for oil fouling, carbon, kinks, and incorrect routing. A blocked reference makes a good sensor read wrong.
8.Verify the sensor's 5-volt reference and ground quality, measuring the ground for low resistance rather than just continuity.
9.With the engine cold and off, move the wastegate linkage by hand. Free travel with light spring resistance is normal; a wastegate stuck open explains a real boost shortfall.
10.Check the wastegate actuator diaphragm by applying vacuum with a hand pump and watching whether it holds.
11.Check the air filter and intake ducting for restriction, and listen for an exhaust leak upstream of the turbine.
12.On modified vehicles, establish whether the calibration has been changed. A stock plausibility table will object to raised boost targets even with healthy hardware.
13.Replace the sensor only after the air side, the reference path, the reference voltage, and the ground have all been verified good.

Repair cost

$110 – $1,400

This code has an unusually wide honest range because it is a plausibility fault rather than a component fault. If the sensor is genuinely bad, parts are typically $60-$250 with 0.3-2 hours of labor depending on how buried the "B" position is, so $110-$450 is the common outcome. A cracked charge pipe, loose clamp, or failed coupler can be under $150. Diagnosis frequently runs $100-$200 because comparing the sensor against a mechanical gauge under load takes real bench time. A charge air cooler replacement generally lands between $400 and $1,200, and wastegate actuator work runs $300-$1,000. The upper end reflects those genuine air-side repairs, not the sensor. Turbocharger replacement is a much larger expense that this code alone rarely justifies.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.