A boost sensor can be wired correctly and still report pressure that cannot be right. A P0240 code means the signal from turbocharger boost sensor B does not match what the engine computer expects.
This guide explains how the reading is checked, why boost leaks and clogged sensor ports cause this code and how to find the fault.
P0240 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 2 | Fuel and air metering | The fault involves fuel and air metering, such as injectors, the fuel pump, throttle sensors or boost control. |
| 40 | Specific fault | Turbocharger Boost Sensor B Circuit Range/Performance |
What the P0240 Code Means
The SAE J2012 standard defines the P0240 code as Turbocharger Boost Sensor B Circuit Range/Performance. Sensor B is a second boost pressure sensor. It is used on some twin turbo engines, on engines with two charge air paths and on some diesels that measure pressure before and after the intercooler. Check your service manual to see where sensor B sits on your engine.
The sensor gets a 5-volt reference and a ground from the powertrain control module (PCM) and sends back a voltage that rises with pressure, usually between about 0.5 and 4.5 volts. With the engine off, it should read the same as barometric pressure, about 14.7 psi or 101 kPa at sea level.
The PCM uses the reading to control boost through the wastegate or turbo vanes and to set fuel delivery. When it cannot trust the sensor, it usually limits boost and puts the engine in reduced power.
Range/performance means the circuit is electrically intact, but the boost sensor B reading does not make sense. It may be stuck, change too slowly, drift or disagree with other inputs the PCM uses to check it.
Symptoms of a Bad Boost Sensor B
Because boost control depends on this sensor, power usually suffers. Common signs of a P0240 code include:
- Check engine light on
- Reduced power or limp mode
- Slow or no turbo boost
- Black smoke on diesels
- Poor fuel economy
- Boost gauge reading wrong or stuck
Some engines run almost normally at light throttle and only show the problem under load, when the PCM needs accurate boost readings the most.
Common Causes of the P0240 Code
Range codes often come from the sensor or the system it measures, not the wiring:
- Boost leak. A loose clamp or split intercooler hose lowers real boost.
- Clogged sensor port. Oil and soot from the crankcase vent can block the sensor tip.
- Failing sensor. The output drifts and no longer matches real pressure.
- Turbo or wastegate problem. Actual boost really is wrong.
- Wrong replacement sensor. A sensor with a different pressure range reads wrong.
- Corroded connector. Resistance shifts the signal voltage.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool that shows boost and barometric pressure
- Digital multimeter with back-probe pins
- Boost leak tester or smoke machine
- Sensor-safe cleaner
- Wiring diagram for your engine
Step-By-Step Diagnosis of the P0240 Code
Work from the scan tool to the connector, and check for boost leaks before you condemn the sensor.
Check Live Data
With the key on and engine off, look at the boost sensor B reading. It should be close to barometric pressure. Then watch it during a brief, safe acceleration. A reading stuck near 0 or 5 volts confirms a circuit fault.
Compare with Barometric Pressure
With the key on and the engine off, compare the boost sensor reading with the barometric pressure reading. They should match within about 1 to 2 psi. A sensor that reads far off at rest is faulty or its port is clogged.
Check for Boost Leaks and a Clogged Port
Remove the sensor and look at its tip for oil and soot. Clean it with sensor-safe cleaner if allowed. Pressure test the intake and intercooler with a boost leak tester or smoke machine, since a leak makes real boost low.
Check for Related Codes
Codes such as P0299 (underboost) or P0106 (MAP range) can point to a shared cause. Fix circuit codes on shared references first, since one shorted sensor can affect several readings.
Repairs That Clear the P0240 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Clean or replace the connector if pins are corroded or loose.
- Repair damaged wiring near the intake and turbo, where heat is high.
- Clean the sensor port if oil or soot blocks it.
- Fix boost leaks by tightening clamps and replacing split hoses.
- Replace boost sensor B if it reads wrong at rest or fails electrical tests.
After the repair, clear the codes and compare the key-on reading with barometric pressure again. Then drive and watch desired and actual boost under load. They should track closely, and the engine should pull normally again.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Assuming a range code always means a bad sensor
- Skipping the mechanical checks that can make a good sensor look wrong
- Replacing the sensor without checking for a clogged port
- Buying a sensor with the wrong pressure range for your engine
Typical Repair Costs
The cost to fix a P0240 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Connector or pigtail | $10 to $40 | $75 to $200 |
| Wiring repair | $10 to $30 | $100 to $300 |
| Boost sensor | $30 to $150 | $150 to $350 |
| Intercooler hose or clamp | $15 to $100 | $100 to $300 |
Most repairs for boost sensor codes are inexpensive. The sensor usually bolts on with one or two screws, so labor is low unless it sits under other parts.
Is It Safe to Drive with a P0240 Code?
You can usually drive gently, since most engines limit boost and run in reduced power. Expect slow acceleration, which can make merging and passing unsafe.
Avoid hard acceleration and towing until the fault is fixed. A wrong boost reading can also cause rich running and heavy smoke on diesels.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0239 | Turbocharger Boost Sensor B Circuit Malfunction |
| P0241 | Turbocharger Boost Sensor B Circuit Low Input |
| P0242 | Turbocharger Boost Sensor B Circuit High Input |
| P0234 | Turbo/Super Charger Overboost Condition |
| P0299 | Turbo/Super Charger A Underboost Condition |
Fixing Turbo Boost Sensor B
A P0240 code means boost sensor B reads pressure the PCM does not believe. Clogged ports, boost leaks and drifting sensors cause most cases.
- Compare the reading with barometric pressure at rest.
- Clean a clogged sensor port.
- Pressure test for boost leaks.
- Replace the sensor if it still reads off.
Frequently Asked Questions
Can a Boost Leak Cause the P0240 Code?
Yes. A split intercooler hose or loose clamp lets boost escape, so the sensor reads less pressure than the PCM expects. The sensor is working correctly, but the reading does not match. Fix the leak before you replace the sensor.
Where Is the Turbo Boost Sensor?
It is usually on the intake manifold or on the charge pipe after the intercooler. On many engines, it is combined with the intake air temperature sensor. Your service manual or a parts catalog shows the exact location.
Is the Boost Sensor the Same as the MAP Sensor?
On some engines, yes. The MAP sensor on a turbo engine often doubles as the boost sensor. Other engines have both, with the boost sensor before the throttle and the MAP sensor after it. Check the part number before buying.
How Do I Test a Boost Sensor with a Multimeter?
Back-probe the connector with the key on. The reference should read about 5 volts and the ground close to 0 volts. The signal should match the pressure chart in your service manual, and it should rise smoothly if you apply gentle pressure to the sensor port with a hand pump.
Can a Bad Boost Sensor Cause Black Smoke?
On diesels, yes. If the sensor reports more boost than the turbo is making, the PCM may add fuel for air that is not there. The extra fuel cannot burn fully, so the engine smokes black and fuel economy drops.
Can Oil Clog the Boost Sensor?
Yes. Crankcase vent oil and soot travel through the intake on many turbo engines and can coat the sensor tip. A coated sensor reacts slowly or reads wrong. Cleaning it carefully, or replacing it, often fixes the reading.