Which sensor is boost sensor "B"?

It depends entirely on the engine, and this is the part worth getting right before you buy anything. On a twin-turbo engine, "B" is usually the sensor for the second turbo or the second bank. On a sequential system it is often the sensor managing the handover between the small and large turbos. On many modern turbodiesels the two sensors sit on either side of the charge air cooler, and "B" is the post-cooler one. Only the service data for your specific engine will tell you which arrangement applies, so look it up rather than assuming.

My engine has one turbo. Can it still set P0239?

Yes, and this catches people out. A second boost sensor does not require a second turbocharger. Many single-turbo diesels fit two pressure sensors — one before the charge air cooler and one after — so the module can confirm the cooler is doing its job and adjust fueling for the resulting air density. That said, if your engine genuinely has only one boost pressure sensor, the code is likely a scan tool misreading a manufacturer-specific code, and re-scanning with a tool that supports your make is the right first move.

How is P0239 different from P0237 and P0238?

Two ways. First, the sensor: P0237 and P0238 refer to sensor "A", while P0239 refers to the second sensor, "B". Second, the specificity: P0237 says the signal is stuck low and P0238 says it is stuck high, which points diagnosis in a particular direction, whereas P0239 is the general form and simply says the circuit is faulty without classifying which way. That means P0239 asks you to check the whole circuit — reference, ground, signal, connector, and any reference hose — rather than narrowing to one likely short.

Is it safe to drive with P0239?

For short distances, usually. Where the second sensor is used only for monitoring, you may notice nothing beyond the light. Where it is part of boost control on a twin-turbo or sequential system, the PCM will normally cap boost or enter a reduced-power mode, so the car feels sluggish but is safe to drive gently to a shop. The reason not to leave it is that these systems are expensive to repair when they run mismanaged, and the code can also be pointing at a real charge air cooler or piping leak rather than an electrical fault. Get it diagnosed rather than clearing it.

OBD-II trouble code

P0239: Turbocharger/Supercharger Boost Sensor "B" Circuit Malfunction

A general fault in the second boost pressure sensor circuit. Sensor "B" only exists on engines with more than one boost sensor — twin-turbo, sequential-turbo, and many modern diesels — so the first job is identifying which physical sensor your engine calls "B".

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 – $1,000
DIY difficulty: Intermediate DIY

What does P0239 mean?

P0239 reports a general circuit malfunction at boost pressure sensor "B". The word that carries the weight here is "B". Where P0235 through P0238 concern sensor "A", this code concerns a second boost pressure sensor, and second sensors only exist on engines that need them.

There are three common reasons an engine carries more than one boost sensor. Twin-turbo engines often fit one per turbo or one per bank so the PCM can see whether both are contributing equally. Sequential systems use a second sensor to manage the handover between the small turbo that spools early and the large one that takes over at higher load. And many modern turbodiesels place one sensor before the charge air cooler and a second after it, so the module can calculate the temperature drop across the cooler, verify the cooler is working, and compensate fueling accordingly. Identifying which of these arrangements your engine uses is the first diagnostic step, because "B" is not a fixed physical location — it is whatever the manufacturer assigned, and the service data for your specific engine is the only reliable source.

The general "circuit malfunction" wording means the module has seen an implausible or out-of-range signal without classifying it as specifically low or high, the way P0237 and P0238 do for sensor "A". In practice the causes are the same family: a short to ground or to voltage, an open in the signal wire, a corroded or water-intruded connector, a lost or high-resistance ground, a missing 5-volt reference, or a sensor that has failed internally. On hose-referenced sensors, a cracked, pinched, or misrouted reference line will also do it.

What is distinctive about a "B" sensor fault is that you frequently have a built-in reference. If sensor "A" is healthy and the two sensors normally see comparable pressure, you can compare them directly in live data. With the key on and the engine off, both should report roughly barometric pressure — any meaningful disagreement at rest is a strong indicator without needing to drive the car at all. Where the two sensors sit on opposite sides of a charge air cooler, they will legitimately differ under load, so use the key-on-engine-off comparison rather than the under-load one to judge plausibility.

The consequence for the driver depends on how heavily the strategy leans on that second sensor. Where it is used for cooler efficiency monitoring, the effect may be limited to a check engine light and some fueling compensation. Where it is part of boost control on a twin or sequential system, the PCM will usually take the conservative route and cap boost or drop into a reduced-power mode, because managing two turbochargers on one trustworthy signal is not something it will attempt. Either way the fault is worth resolving rather than clearing, since these systems are expensive when they are allowed to run mismanaged.

Common causes

Failed boost pressure sensor "B"
Corroded, loose, or water-intruded connector at the second boost sensor
Open, shorted, or chafed wiring in the sensor "B" circuit
Lost or high-resistance sensor ground
Missing or incorrect 5-volt reference at the sensor
Cracked, pinched, oil-soaked, or misrouted pressure reference hose
Wrong-range or incorrect replacement sensor fitted to the "B" position
Charge air cooler or piping leak producing an implausible pressure differential between the two sensors
PCM internal fault on that sensor input, rare and diagnosed last

Symptoms

Check engine light with P0239 stored
Reduced power or a limp mode capping boost, particularly on twin-turbo and sequential systems
Hesitation or an uneven handover between turbos on sequential setups
Erratic or implausible boost values in live data
Poor fuel economy
Black smoke on diesel applications where fueling compensation is affected
Sometimes no driveability symptoms at all where the sensor is used only for monitoring

Diagnostic steps

1.Establish which physical sensor your engine calls boost sensor "B" using service data for that specific engine. On twin-turbo engines it is often the second turbo or second bank; on many diesels it is the post-intercooler sensor.
2.With the key on and the engine off, compare sensor "A" and sensor "B" in live data. Both should report roughly barometric pressure. Disagreement at rest is a strong indicator of a sensor or circuit fault without any driving.
3.Note whether sensor "A" codes are also stored. Faults on both point at something shared — a common reference voltage, ground, or harness section — rather than two failed sensors.
4.Inspect the sensor "B" connector for corrosion, water intrusion, and pushed-out or spread pins.
5.Verify the 5-volt reference and the ground at the sensor connector, checking the ground for low resistance rather than just continuity.
6.Check the signal wire for shorts to ground and to voltage, and inspect the harness run for chafing.
7.On hose-referenced sensors, confirm the reference line is correctly routed for the "B" position and is not cracked, pinched, or oil-soaked.
8.Back-probe the signal wire and watch for dropouts while wiggling the harness, since a general circuit malfunction is often intermittent.
9.Where the two sensors straddle the charge air cooler, inspect the cooler and its piping for leaks that would produce an implausible differential.
10.Replace the sensor only after the reference, ground, signal wire, connector, and reference hose have all been verified good.

Repair cost

$110 – $1,000

The sensor itself typically runs $60-$250 in parts. Labor varies widely by where the "B" sensor lives — a sensor in accessible charge piping is a 20-minute job, while a post-intercooler sensor buried behind bumper structure or under the intake can take two hours or more. Wiring, ground, and connector repairs often land between $110 and $250. Diagnosis commonly runs $100-$180 because identifying which sensor is "B" and comparing the two in live data takes more than a code read. The upper end covers cases where a chafed harness must be traced properly, or where the code turns out to reflect a genuine charge air cooler or piping leak that needs its own repair.

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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.