Drivable short-term, but expect limited power and boost while the PCM runs a default strategy. Repair soon so normal boost control returns and an unnoticed leak or boost-control fault does not stress the engine. P0240 is a range/performance fault for the turbocharger (or supercharger) boost-pressure sensor 'B' — the second boost sensor on engines that use more than one. The powertrain control module (PCM) has decided sensor B's reading is implausible: it does not agree with barometric pressure, the MAP sensor, or the boost the engine should be making. It is the 'B' counterpart to the sensor-A family (P0235 circuit, P0236 range/performance, P0237 low, P0238 high).
What P0240 means
A forced-induction engine uses a boost-pressure sensor to report how much pressure the turbo or supercharger is building in the intake tract. Some engines — twin-turbo, sequential-turbo, or diesels with a second charge-pressure measurement point — carry two such sensors, designated 'A' and 'B'. Each is typically a three-wire pressure device fed a 5-volt reference and a shared ground, returning a signal that climbs with pressure; on some units it is a combined pressure-and-temperature sensor. P0240 is the rationality (range/performance) code for sensor B: unlike a hard circuit fault, the wiring may be electrically intact, but the value the PCM sees is not believable. Factory logic sets this family when the boost sensor reading fails to correlate with barometric pressure at key-on/engine-off, fails to match the manifold-pressure (MAP) sensor at idle, or fails to track actual boost while driving. Because the PCM cannot trust sensor B for closed-loop boost control, it usually falls back to a default strategy and caps boost, dropping the engine into reduced-power/limp operation until the fault clears.
Symptoms
- Check-engine light on, often stored in continuous (keep-alive) memory
- Reduced engine power or a limp/fail-safe mode that limits boost
- Sluggish acceleration and soft throttle response, most obvious under load
- Weak, erratic, or inconsistent boost even though the turbo/supercharger seems to spin up
- Boost-pressure readings on a scan tool that disagree with the MAP or barometric values
- Possible surge, hesitation, or uneven power delivery when accelerating
- Sometimes paired with sibling boost-sensor codes (P0235/P0236/P0237/P0238) or wastegate codes
Common causes
- Contaminated, blocked, or oil/soot-fouled boost sensor 'B' giving a slow or offset reading
- Slow-responding or drifting boost sensor 'B' that no longer tracks real pressure
- Intake-air leaks, loose charge-air (intercooler) tube clamps, or a split boot upstream of the sensor
- Restricted intake system, clogged air filter, or a partially blocked charge-air cooler skewing the reading
- Damaged or failed boost sensor 'B' element
- Corroded, loose, or moisture/oil-fouled sensor connector with spread or pushed-back terminals
- High-resistance or intermittently open signal, 5V reference, or ground wire shifting the value out of range
- Actual boost-control fault (sticking wastegate, leaking hose, or turbo problem) making the honest reading look implausible
- Rarely, a PCM software/calibration issue or a shared-reference problem affecting sensor B
Severity & driving advice
Severity: Moderate — Not an immediate hazard, but the engine usually drops into reduced-power limp mode with capped boost until sensor B's reading is trusted again.
Can I drive? Drivable short-term, but expect limited power and boost while the PCM runs a default strategy. Repair soon so normal boost control returns and an unnoticed leak or boost-control fault does not stress the engine.
Diagnostic approach
- Scan all codes and capture freeze-frame — Retrieve every stored and pending code and record the freeze-frame conditions (RPM, load, boost, coolant temp) when P0240 set. Note whether sibling boost-sensor codes (P0235-P0238) or wastegate codes are also present, since range/performance means the circuit may be electrically fine while the value is wrong — a genuine boost-control fault can trip it just as easily as a bad sensor.
- Compare boost sensor B against BARO and MAP with the key on, engine off — With the ignition on and engine off, all pressure sensors should read close to barometric pressure. Watch the boost sensor 'B' PID next to the BARO and MAP PIDs; if sensor B does not sit within a small tolerance of the others at rest, its rationality has failed. This mirrors the factory set condition, where the boost-pressure signal must correlate with BARO at key-on/engine-off and with MAP at idle.
- Watch sensor B track live boost while driving — Under load, sensor B's reported pressure should rise smoothly with commanded and actual boost and stay in step with sensor A and the MAP PID. A signal that lags, flat-lines, reads consistently high or low, or diverges from the other sensors as boost builds confirms a performance fault rather than a genuine over- or under-boost event.
- Inspect the intake and charge-air system for leaks and restrictions — Check the intake tract, charge-air (intercooler) tubing, clamps, and boots between the compressor and throttle body for leaks, cracks, or loose connections, and confirm the air filter and charge-air cooler are not restricted. A leak or restriction near sensor B makes an honest reading look implausible; factory guidance calls out inspecting the intake air system for leaks and restrictions before condemning the sensor.
- Inspect the sensor, connector, and wiring — Locate boost sensor 'B' on the charge-air tube (on combined units it is a pressure/temperature sensor). Unplug it and check for corrosion, oil, moisture, or bent/spread terminals, and inspect the harness for chafing or pinching. Wiggle-test the connector and leads while watching live data to expose an intermittent connection that can push the value out of range.
- Verify the 5V reference, ground, and signal — Key on, back-probe the connector and confirm roughly 5 volts on the reference wire and a clean ground near 0 V. A high-resistance reference or ground can offset the signal enough to fail rationality without setting a hard low/high code. Check continuity of the signal, reference, and ground circuits back toward the PCM to rule out an open or high-resistance run.
- Confirm the sensor and rule out a real boost-control fault — If wiring, reference, ground, and the intake system all check out, test the sensor against a known applied pressure or substitute a known-good unit and confirm the signal moves smoothly and accurately. Also verify the wastegate/turbo is actually delivering commanded boost, so you do not replace a good sensor that is faithfully reporting a mechanical boost problem.
- Repair, clear, and road-test — Correct the fault found — replace a contaminated or slow sensor, repair a leak or restriction, or fix a connector/wiring issue. Clear the code and drive through a full boost cycle while monitoring sensor B against BARO, MAP, and sensor A. Confirm the readings correlate, fail-safe mode no longer engages, and recheck continuous memory to be sure P0240 does not return.
FAQ
What does the 'B' in P0240 mean?
The 'B' identifies the second turbocharger/supercharger boost-pressure sensor. Engines with more than one boost sensor — twin-turbo, sequential-turbo, or certain diesels — label them 'A' and 'B'. P0240 concerns sensor B specifically, while P0235-P0238 cover the equivalent sensor-A faults. It does not refer to a cylinder or a bank.
Does P0240 mean my turbocharger is bad?
Usually not directly. P0240 is a range/performance (rationality) code, meaning sensor B's reading is implausible. Most often that traces to a contaminated or slow sensor, an intake/charge-air leak or restriction, or a wiring/connector problem. A genuine boost-control fault (sticking wastegate or turbo) can also make an honest reading look wrong, so verify actual boost before replacing hardware.
What is the difference between P0240 and P0235/P0236/P0237/P0238?
P0235-P0238 all describe the boost-pressure sensor 'A' circuit: P0235 is the general circuit fault, P0236 range/performance, P0237 circuit low, and P0238 circuit high. P0240 is the range/performance fault for the second sensor, 'B' — the same type of implausible-reading problem, just on sensor B instead of sensor A.
How does the PCM decide the boost reading is out of range?
It cross-checks the boost sensor against other pressure references. At key-on/engine-off the boost sensor should read close to barometric pressure; at idle it should agree with the MAP sensor; and while driving it should track actual boost. If sensor B fails to correlate with BARO, MAP, or the other boost sensor, the PCM flags a range/performance fault and sets P0240.
Can I keep driving with P0240?
You can usually drive short distances, but the engine typically enters reduced-power fail-safe with limited boost, so it will feel sluggish. It is not an immediate safety hazard, but repair it soon — an untraced intake leak or boost-control problem behind the code can worsen or hurt performance and economy.
How do I test the boost-pressure sensor for P0240?
With a scan tool, compare sensor B to BARO and MAP with the key on, engine off — they should read nearly the same. Confirm it tracks boost smoothly under load alongside sensor A. Then back-probe the connector for about 5 volts of reference and a clean ground, and check the intake/charge-air system for leaks and restrictions. If wiring and induction are good but the value is still implausible, the sensor is the likely fault.