P0238 Code: Turbocharger/Supercharger Boost Sensor A Circuit High
The boost sensor signal is stuck near 5 volts, so the computer sees impossibly high boost and cuts power.
At a glance
Severity: Moderate
- Meaning
- Turbocharger/Supercharger Boost Sensor A Circuit High
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $40–$550
- DIY difficulty
- Moderate
A boost pressure sensor normally sends between roughly 0.5 and 4.5 volts depending on pressure. P0238 sets when the signal sits above the top of that window, usually 4.8 volts or more, for longer than a moment. To the engine computer that looks like extreme boost even at idle, so it opens the wastegate or vanes fully, limits fuel and often goes straight into limp mode, which makes the engine feel very weak. The cause is almost always electrical: a sensor whose output has failed high, a broken ground wire that lets the signal float up to the reference voltage, or the signal wire touching the 5-volt feed inside a damaged connector or melted harness.
Can I keep driving? The engine is safe to run because the computer protects it by cutting boost, but power is poor. Drive gently to a shop, and avoid situations where you'd need full acceleration.
How to read P0238
- P Powertrain
- 0 Generic (SAE)
- 2 Fuel & air (injector circuit)
- 38 Specific fault
Symptoms
- Check engine light and immediate limp mode, often right at start-up
- Boost reading showing a very high value even with the engine idling
- Very little power, turbo seems completely off
- Sometimes an overboost code stored alongside
- Aftermarket boost gauge (if tapped into the sensor) reading odd or maxed out
- Fault stays present after a restart, unlike most intermittent issues
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Boost pressure sensor output failed high internally
- Open or corroded sensor ground circuit letting the signal float up
- Signal wire shorted to the 5-volt reference in the connector or harness
- Signal wire shorted to battery voltage (chafed against a powered wire)
- Wrong sensor fitted, with a different voltage scale
- Aftermarket gauge or module spliced into the signal wire
- Engine computer input fault (rare)
Diagnosis step by step
Read the raw voltage
With the key on, engine off, display the boost sensor voltage if your tool offers it. A value of 4.8 to 5.0 V, or a pressure reading at the top of the sensor's range such as 36 to 44 psi absolute on a 2.5- or 3-bar sensor, confirms the stuck-high signal. A healthy sensor would read about 1.0 to 2.0 V here.
Tools: OBD-II scanner
Unplug the sensor and watch the value
Disconnect the sensor with the key on. The computer pulls an open signal line low, so the reading should drop to minimum and a low-circuit code may appear. If the value stays high with the sensor unplugged, the signal wire is shorted to a voltage source somewhere in the harness.
Tools: OBD-II scanner
Check the ground wire
Back-probe the ground pin and battery negative with the key on and the sensor plugged in. More than about 0.1 V means resistance in the ground path. An open ground makes many sensors drive their output right up to the reference voltage, which looks exactly like P0238.
Tools: Multimeter
Look for a short between signal and reference
With the battery disconnected and both the sensor and computer connectors unplugged, measure resistance between the signal and 5-volt reference wires. It should be open, well over a megohm. Any continuity means the wires touch, often where the harness passes a hot charge pipe or has been pinched under a bracket.
Tools: Multimeter
Check for battery voltage on the signal wire
With the sensor unplugged, measure the signal wire to ground with the key on. Anything above 5 V, especially 12 V, means a short to a switched power circuit and must be fixed before fitting a new sensor, or it may damage the replacement and the computer input.
Tools: Multimeter
Fit the correct sensor and confirm
Verify the replacement matches the original part's pressure range and connector. After fitting it, key-on voltage should fall back into the 1 to 2 V range and rise smoothly under boost. Clear the code and confirm the engine pulls normally.
Tools: OBD-II scanner, Multimeter
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Repair broken ground wire or clean ground point | No | $40–$200 |
| Replace boost pressure sensor | Yes | $80–$320 |
| Replace damaged sensor connector | No | $60–$220 |
| Repair shorted harness section | No | $150–$550 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
With the check engine light on, the vehicle fails OBD-II emissions testing. This code tends to stay active constantly, so it won't clear on its own before a test.
Frequently asked questions
If the computer thinks boost is too high, is my engine in danger?
No, it's the other way around. The boost isn't really high; the signal is. Because the computer believes there's too much boost, it opens the wastegate or vanes and cuts power, which protects the engine but leaves you with very little performance.
Why is it usually not the sensor with a high code?
A failed sensor is still the most common single cause, but an open ground or a short to the 5-volt feed produces the same stuck-high voltage. The unplug test and a ground check take minutes and tell you which it is.
Can a boost gauge I installed cause P0238?
Yes. Some electronic gauges tap into the factory sensor's signal wire, and a poor splice or a gauge that adds its own voltage can pull the signal high. Disconnect the gauge wiring and see whether the code returns.
What's the difference between P0238 and P0234?
P0234 means the computer believes real boost is too high, based on a plausible sensor signal. P0238 means the signal itself is out of range at the top. If both appear together, check the sensor circuit first.