OBD-II trouble code
P0242: Turbocharger/Supercharger Boost Sensor "B" Circuit High
The signal from the second boost pressure sensor has risen above its valid range. A short to voltage will do it, but so will a lost or high-resistance ground — and because the module now believes boost is excessive, this code tends to cut power harder than its circuit-low counterpart.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $950
- DIY difficulty
- Intermediate DIY
What does P0242 mean?
P0242 sets when the signal voltage returning from boost pressure sensor "B" climbs above the highest value the module considers physically credible — usually somewhere close to the 5-volt reference itself. In pressure-sensor terms, a high signal voltage means high pressure, so what the module is effectively receiving is a report of boost that cannot possibly be there.
The obvious cause is a short to voltage. If the signal wire touches the reference wire, an ignition feed, or a battery feed anywhere along its run, the module sees that voltage instead of the sensor's output. What makes this cause tractable is that it is physical: a wire cannot short to voltage without touching something energized, so there is damage to find. Follow the harness, look for chafed insulation, flattened sections, melted spots where it has rested against something hot, and points where it passes through a bracket or bulkhead. Wiggle-testing while watching live data will usually reveal it.
The cause people miss is the ground. A boost pressure sensor's output is referenced between the 5-volt supply and its ground. If the ground is lost, or has developed enough resistance through corrosion to matter, nothing is holding the signal down and it drifts up toward the reference voltage. The module reads that drift as high pressure and sets a circuit-high code, even though the sensor itself is in perfect condition. This is worth testing properly rather than casually: measure the ground for low resistance to a known-good ground point under a small load, because a corroded ground will often show continuity on a meter while failing completely once current flows.
A third possibility on hose-referenced installations is contamination in the pressure reference line. Oil, water, or carbon accumulating in the line or in the sensor's port can hold pressure against the diaphragm and produce a persistently elevated reading. On diesel and direct-injection applications this happens more than it should, and the fix is cleaning or replacing the line rather than the sensor.
The practical difference between this code and P0241 is worth understanding, because it changes the symptoms. When the signal is stuck low, the module thinks there is no boost and generally caps output conservatively. When the signal is stuck high, the module thinks the engine is overboosting — and overboost is the condition it protects against most aggressively, since it can damage pistons, head gaskets, and the turbocharger itself. So the response is usually more decisive: fuel pulled, timing retarded, boost commanded down hard, and often an immediate drop into reduced power. Drivers frequently describe P0242 as the car losing power suddenly and completely rather than just feeling flat.
As with every "B" code, establish which physical sensor that is on your engine before you start. Twin-turbo engines commonly assign one sensor per turbo or per bank, sequential systems use the second sensor to manage the handover, and many single-turbo diesels place one sensor before the charge air cooler and one after it. If sensor "A" is healthy, use it: with the key on and the engine off, both should report roughly barometric pressure, and "B" reading high in that state is immediate confirmation the fault is real and local.
Common causes
- Signal wire shorted to voltage — to the 5-volt reference, an ignition feed, or a battery feed
- Lost or high-resistance sensor ground allowing the signal to drift toward the reference voltage
- Corroded sensor ground connection or a loose ground eyelet
- Chafed, flattened, or heat-melted harness insulation where it rests against a hot or sharp surface
- Corroded or water-intruded connector at the second boost sensor
- Oil, water, or carbon contamination in the pressure reference line or sensor port holding pressure on the diaphragm
- Failed boost pressure sensor "B" reading high internally
- Wrong-range or incorrect replacement sensor fitted to the "B" position
- Genuine overboost, though that normally sets P0234 rather than a circuit code
- PCM internal fault on that sensor input, rare and diagnosed only after everything else checks out
Symptoms
- Check engine light with P0242 stored
- Sudden and pronounced loss of power, often more abrupt than with the circuit-low code
- Reduced-power or limp mode
- Boost pressure reading implausibly high in live data, including at idle or with the key on and engine off
- Timing retarded and fuel pulled, felt as flat, unresponsive throttle
- Poor fuel economy
- Hesitation or a rough handover on sequential-turbo systems
- Additional sensor codes at the same time if a shared ground has failed
Diagnostic steps
- 1.Record all stored codes. Several sensor codes together often means a shared ground has failed rather than one sensor.
- 2.Identify which physical sensor your engine calls boost sensor "B" from service data before touching anything.
- 3.With the key on and the engine off, read both boost sensors in live data. Both should show roughly barometric pressure. "B" showing a high value with the engine not running is immediate confirmation.
- 4.Measure the sensor ground for low resistance to a known-good ground point. Test it under a small load rather than trusting a simple continuity check, since corroded grounds pass continuity tests and still fail in service.
- 5.Unplug the sensor and check the signal wire with the key on. If it still reads high with the sensor disconnected, the fault is a short to voltage in the harness, not the sensor.
- 6.Check for a short between the signal wire and the 5-volt reference wire, which is the most common short-to-voltage path on a three-wire pressure sensor.
- 7.Inspect the connector for corrosion, water intrusion, and spread or pushed-in terminals.
- 8.Follow the harness run looking for chafed insulation, flattened sections, and melted spots, paying particular attention to anywhere it passes the turbocharger, exhaust manifold, or a bracket edge.
- 9.On hose-referenced sensors, remove the reference line and check it for oil, water, and carbon. Clean or replace it if contaminated, and check the sensor port as well.
- 10.Wiggle-test the harness while watching live data, since a short to voltage is frequently intermittent and position-dependent.
- 11.Confirm against a mechanical boost gauge if there is any chance boost really is high, though genuine overboost normally sets P0234.
- 12.Replace the sensor only after the ground, the signal wire, the reference, the connector, and the reference line have all tested good.
Repair cost
$100 – $950
The sensor typically runs $60-$250 in parts, and labor varies with how buried the "B" position is — accessible charge piping is a 20-minute job, a post-intercooler sensor behind bumper structure can take two hours. That puts the common repair between $100 and $450. Cleaning a contaminated reference line is often under $120. Ground repairs are frequently the cheapest real fix at $80-$200, since it may be nothing more than cleaning and re-torquing a corroded eyelet. Tracing and repairing a short to voltage generally lands between $150 and $500 because the cost is the labor of finding it. Diagnosis commonly runs $100-$180.
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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.