When a boost sensor signal climbs close to 5 volts, it reports more boost than the turbo could ever make. A P0242 code means the turbocharger boost sensor B signal is higher than its allowed range.
This guide explains what pushes the signal high, why the ground is the first thing to test and how to fix it.
P0242 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. |
| 42 | Specific fault | Turbocharger Boost Sensor B Circuit High Input |
What the P0242 Code Means
The SAE J2012 standard defines the P0242 code as Turbocharger Boost Sensor B Circuit High Input. 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.
High input means the signal voltage rose above the highest value a working boost sensor B should ever send. That usually points to an open ground, a signal wire touching the reference or another voltage source, or a failed sensor.
Symptoms of a Bad Boost Sensor B
Because boost control depends on this sensor, power usually suffers. Common signs of a P0242 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.
What Causes the P0242 Code?
High input faults almost always come from an electrical problem:
- Open sensor ground. Without a ground, the signal floats up toward the reference.
- Signal wire shorted to the 5-volt reference. Damaged insulation lets the wires touch.
- Signal wire shorted to battery voltage. Less common, but possible.
- Corroded connector. Resistance on the ground pin raises the signal.
- Failed boost sensor B. An internal fault locks the output high.
- PCM fault. Rare.
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
How to Track Down the P0242 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.
Test the Ground First
Back-probe the sensor ground with the key on and measure against the battery negative post. It should read close to 0 volts, ideally under 0.1 volt. A higher reading means a poor ground.
Check for a Short to Voltage
Unplug the boost sensor B with the key on and watch the signal in live data. If it stays high with the sensor unplugged, the signal wire is shorted to the reference or another voltage source. If it changes, the sensor itself is likely at fault.
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.
How to Fix the P0242 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:
- Replacing the sensor without testing the ground first
- Overlooking a short to voltage inside a wiring bundle
- 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 P0242 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 Keep Driving with a P0242 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 |
| P0240 | Turbocharger Boost Sensor B Circuit Range/Performance |
| P0241 | Turbocharger Boost Sensor B Circuit Low Input |
| P0234 | Turbo/Super Charger Overboost Condition |
| P0299 | Turbo/Super Charger A Underboost Condition |
Fixing Turbo Boost Sensor B
A P0242 code means boost sensor B voltage is too high. Lost grounds, shorts to voltage and failed sensors cause most cases.
- Confirm the high reading in live data.
- Test the sensor ground first.
- Unplug the sensor to check for a short to voltage.
- Replace the sensor only after the wiring tests good.
Frequently Asked Questions
Can a Bad Ground Cause the P0242 Code?
Yes. Without a solid ground, the sensor signal floats up toward the 5-volt reference and looks like extreme boost. Check the ground pin and wire closely before replacing 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.