P0239 Code: Turbocharger/Supercharger Boost Sensor B Circuit

What the P0239 code means

On twin turbo engines, or engines with two measuring points, sensor B measures the pressure of the second turbo or of another section of the intake. If its signal is not valid, the computer limits boost.

The P0239 code is stored when the computer detects a general electrical fault in the circuit. The sensor works with a 5 volt reference, a ground and a signal that should vary between about 0.5 and 4.5 volts. The extreme values, near 0 volts or near 5 volts, are reserved for detecting faults. A signal that sits at either extreme does not match any real position or pressure.

On twin turbo engines each sensor watches one charge circuit. If one fails, the computer cannot balance the two turbos and it limits both. On other engines sensor B sits before the intercooler and sensor A after it, and the difference between them is used to monitor the intercooler.

Remember that the code tells you where the computer saw the problem, not which part is broken. Use the sensor testing steps below to find the real fault.

P0239 code symptoms

• Check engine light on, and sometimes the power reduced light as well.
• Loss of power because the computer limits the turbo.
• Lack of push when accelerating.
• Other related codes from the same sensor.

Likely causes of the P0239 code

1. Damaged or loose connector. This is the most common cause.
2. Wiring cut or shorted between the sensor and the computer.
3. Failed sensor, open or shorted inside.
4. Missing 5 volt reference. Check whether other sensors are also failing.
5. Computer. Uncommon, so rule out everything above first.

How to diagnose the P0239 code step by step

1. Read the signals on the scan tool and compare them with the other sensors or with the commanded pressure.
2. Inspect the connector. Look for green, backed-out or oily pins and check that the lock is closed.
3. Check 5 volts and ground at the connector with the key on: 4.8 to 5.1 volts on the reference and less than 0.1 volt of drop on the ground.
4. Measure the signal with the connector plugged in while you move the pedal or throttle, or accelerate for a turbo. It should change smoothly, with no jumps and no drops to zero.
5. Do a wiggle test with the engine running. Move the harness and watch the reading.
6. Check continuity of each wire back to the computer with the key off. It should be under 1 ohm.
7. Compare sensors A and B with the engine off, where they should match, and during acceleration, where they should follow a consistent relationship.
8. Check the pressure port of sensor B and its hose. They often clog with oil.

How to fix the P0239 code

• Repair the connector or the wiring.
• Replace the sensor. On electronic throttle systems this is normally the pedal or the whole throttle body.
• Check the hoses and pressure ports of the turbo.
• Clear the codes and confirm the repair with a road test.

Technical reference data

• 5 volt reference, ground and a signal of roughly 0.5 to 4.5 volts.
• Engine off: atmospheric pressure, about 100 kPa (14.5 psi) at sea level.
• At full load: 150 to more than 250 kPa absolute (about 22 to 36 psi), depending on the engine.
• These are reference values only. Check the manufacturer data for your engine.

Brand-specific notes

On diesels, the turbo pressure sensor often includes the intake air temperature sensor, and oil from the crankcase vent can foul it. If you see the P0239 code on a high mileage diesel, clean the port before you spend money on parts.

Frequently asked questions about P0239