P0238 Code: Turbocharger/Supercharger Boost Sensor A Circuit High

What the P0238 code means

The boost pressure sensor tells the computer how much pressure the turbo is delivering. With that number the system calculates fuel injection and runs the mechanism that regulates the turbo.

The P0238 code is stored when the signal goes above the expected maximum. There is an ambiguity here that you should settle before you touch anything, because that reading can come from two very different situations.

The first is an electrical fault. It can be a short between the signal wire and a voltage line or, more often, an open ground that keeps the internal circuit from pulling the voltage down even though the sensor itself is fine.

The second is real overboost. If the mechanism that regulates the turbo is stuck and does not release pressure, boost really does climb above what is intended, and the sensor is reporting it correctly. The way to tell them apart is simple: check the reading with the engine off, when no overboost of any kind is possible.

It helps to know that this sensor and the manifold pressure sensor may be the same part or two different parts, depending on the vehicle. On many turbocharged engines a single sensor covers the whole range, from the vacuum at idle to the boost from the turbo. On others there is one before the intercooler and another after it. Finding out how many your engine has keeps you from testing the wrong one, and it explains why this code sometimes shows up along with MAP sensor codes.

One more detail about location decides where to look. On some vehicles the sensor is threaded straight into the intake manifold and measures there. On others it is mounted away from the manifold and gets pressure through a hose. In that second case the hose is as much a part of the measuring circuit as the wire, and a clog or a kink gives bad readings even though the sensor and the turbo are in perfect shape.

P0238 code symptoms

• Loss of power with the vehicle going into limp mode.
• Check engine light on.
• Sudden power cut under load if there is real overboost.
• Black smoke under acceleration.
• Rough idle.
• An absurd pressure reading on the scan tool.
• Boost codes may be stored along with it.
• Wrong readings with a healthy sensor and turbo, caused by a clogged hose.
• Manifold pressure sensor codes may appear too if the two share a component.

Likely causes of the P0238 code

1. Sensor ground open or with high resistance, which drives the signal to maximum with the sensor intact.
2. Signal wire shorted to a voltage line.
3. Real turbo overboost from a stuck regulating mechanism.
4. Sensor failed internally.
5. Connector with moisture or conductive residue between the pins.
6. Wiring rubbed against a power line.
7. Sensor hose clogged on remote mounted sensors.
8. Turbo actuator out of calibration after a replacement.
9. Pressure port hose kinked or full of carbon on remote mounted sensors.

How to diagnose the P0238 code step by step

1. Check the reading with the key on and the engine off. It should show atmospheric pressure. A high value here confirms the electrical fault, because with the engine off there can be no overboost.
2. If the reading at rest is correct, look for real overboost by comparing commanded and actual pressure under load.
3. Measure the sensor ground and its voltage drop.
4. Unplug the sensor and watch the reading. If the high voltage stays, the problem is in the wiring.
5. Look for shorts between the signal wire and the power lines.
6. Inspect the connector for moisture and conductive residue.
7. Check the turbo regulating mechanism if there is real overboost.
8. Inspect the sensor hose on remote mounted sensors.
9. Find out whether the sensor is threaded into the manifold or mounted remotely. In the second case, check the whole hose, because it is part of the measuring circuit.

How to fix the P0238 code

• Repair the sensor ground down to bare metal and check its voltage drop.
• Fix the short on the signal wire and protect the repaired section.
• Clean the turbo regulating mechanism if it is stuck and causing real overboost.
• Recalibrate or replace the turbo actuator following the manufacturer procedure.
• Dry and clean the connector, and protect it with dielectric grease.
• Replace the sensor if the circuit is good and it still gives an impossible signal.
• Reconnect or replace the sensor hose if it is clogged.
• Replace the pressure port hose if it is kinked, hardened or blocked with carbon.
• Find out how many pressure sensors the engine has before you replace one, because some share a part with the MAP sensor.

Technical reference data

• Typical signal: 0.5 to 4.5 volts depending on pressure.
• With the key on and the engine off it should read atmospheric pressure.
• Typical boost on a modern diesel: 1.0 to 2.2 bar absolute (about 14.5 to 32 psi).
• An open ground drives the signal to maximum even if the sensor is intact.
• P0237 is the opposite code: voltage too low.
• Altitude lowers the atmospheric reference pressure.
• On remote mounted sensors, the pressure hose is part of the measuring circuit.

Brand-specific notes

The check that organizes the whole diagnosis of this P0238 code takes ten seconds: look at the reading with the key on and the engine off. If it already reads high while the turbo cannot possibly be spinning, the problem is electrical and you do not need to touch the turbo.

An open ground is the cause that gets the most sensors replaced for no reason. Measure its voltage drop before you decide.

Frequently asked questions about P0238