P0238
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
You lose power through a derate and you lose the module's ability to detect real overboost, so the engine runs without one of its protections. Most vehicles default to conservative low boost rather than allowing damage, and the power loss usually arrives smoothly rather than as an abrupt cut. How aggressively a given vehicle derates varies, though, so judge drivability by how the vehicle actually behaves rather than by the code. It needs fixing promptly.
- Safe to drive
- Usually drivable if the engine runs smoothly and makes enough power to keep up with traffic — expect reduced performance. Avoid sustained full throttle and heavy loads such as towing or long grades: boost control is running blind and the module can no longer detect a real overboost. If power loss is severe enough that you cannot merge or maintain speed safely, have it towed.
Turbocharger/Supercharger Boost Sensor "A" Circuit High
The boost pressure sensor's signal is reading a voltage higher than the computer will accept. This is an electrical range fault on that sensor's circuit — it is not, by itself, evidence that the engine actually made too much boost.
The boost or charge pressure sensor is a pressure transducer fed by a 5V reference, returning a voltage roughly proportional to absolute pressure in the charge pipe or intake manifold. P0238 sets when that returned voltage sits above the module's fixed ceiling — a level the sensor should not reach at any pressure the engine can generate. The usual causes are the signal line pulled toward supply, an open or high-resistance sensor ground allowing the signal to float up, or a transducer that has failed internally. With no trustworthy boost feedback, boost control becomes open-loop and most vehicles derate power.
How the car detected itRaw signal voltage compared against a fixed upper limit over a short debounce, independent of engine operating point. Because the check is purely electrical, it can set with the key on and the engine not running. That is what distinguishes it from an overboost code, which involves comparing measured pressure against a calculated target. The exact ceiling and debounce vary by vehicle.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Usually drivable if the engine runs smoothly and makes enough power to keep up with traffic — expect reduced performance. Avoid sustained full throttle and heavy loads such as towing or long grades: boost control is running blind and the module can no longer detect a real overboost. If power loss is severe enough that you cannot merge or maintain speed safely, have it towed.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- You keep the derate and lose overboost protection. If a boost control fault develops while the sensor circuit is still faulted, nothing is watching for the resulting overpressure — that combination is how head gaskets and ring lands get hurt. It will also fail an OBD-II emissions inspection: a confirmed fault with the malfunction indicator lamp commanded on is a failure on its own, and clearing the code just before a test swaps that for an incomplete-readiness failure until enough drive cycles have run.
- Check engine light
- Solid
- Significant power loss / limp mode
- Possible blue or black smoke from exhaust
- Whistling or hissing sound from intake
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Sensor circuit shorted to supply voltage | Very common | — |
| 2 | Boost pressure sensor failed | Common | — |
| 3 | Connector pins corroded - high contact resistance | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the code is current or historic
A live fault can be measured directly at the sensor in minutes. A historic one requires wiggle testing and datalogging, which is an entirely different approach.
Sensor reading with the key on and the engine off, compared to barometric pressure
Free and decisive. With the engine stopped, the sensor is measuring ambient air. A reading pegged far above atmospheric confirms the fault is present and narrows it to the sensor or its circuit.
The connector for oil, water, and spread or corroded terminals
Charge pipe sensors sit in a hot area often coated with residue from oil mist or a boost leak. Contamination bridging reference into signal reproduces this code exactly.
Any recent intake, intercooler, or turbocharger work
A sensor left unplugged, a connector not fully latched, or a harness pinched during reassembly is far more likely than a sensor that failed on its own.
Other codes on sensors sharing the same 5V reference
Several sensors reporting implausible values simultaneously means you are chasing a reference or ground problem, not a boost sensor.
Compare the boost sensor to barometric pressure, key on and engine off
Tools Scan tool with live data
Read both on live data with the engine stopped. If the vehicle has more than one pressure sensor, compare them all — they should agree at rest.
ExpectedThe boost/charge pressure sensor should read essentially atmospheric and match the barometric sensor closely. Pegged high confirms the fault is live right now.
Disconnect the sensor, then jumper the signal pin to sensor ground at the harness connector
Tools Scan tool, fused jumper wire, basic hand tools
Unplug the sensor and note what the module reports. Then bridge the signal pin to the sensor ground pin at the harness side with a fused jumper wire and watch the value. The jumper is the part that decides anything — the unplugged reading on its own does not.
ExpectedA high reading with the sensor simply unplugged is normal on most modules and proves nothing — these inputs are usually biased high, which is exactly why an unplugged MAP or boost sensor routinely sets a circuit-high code on a healthy vehicle. With the signal pin jumpered to sensor ground, the reported value should fall to minimum. If it stays pegged high, the signal wire is shorted to voltage or the module input has failed. If it drops to minimum, the wiring and module input are good and the sensor or its ground remains suspect.
Voltage-drop the sensor ground under load
Tools Digital multimeter with backprobes
Backprobe the sensor ground with everything connected and measure to battery negative. Testing an unloaded ground circuit will hide a resistive fault completely.
ExpectedOnly a small drop — a few tenths of a volt at most. An open or resistive ground allows the signal to float toward reference and is a frequently missed cause of circuit-high codes.
Measure the 5V reference at the connector
Tools Digital multimeter
Check it static and while flexing the harness. If it is low, start unplugging the other sensors that share it.
ExpectedSteady and near nominal — a 5V reference should sit close to 5.0V. Which sensors share a given reference circuit is vehicle-specific.
Isolate a short to voltage on the signal wire
Tools Digital multimeter, correct wiring diagram for the vehicle
With the harness disconnected at both the sensor and the module, check the signal wire against battery positive and against the reference wire, and verify continuity end to end.
ExpectedNo continuity or voltage from the signal wire to any supply, and near-zero resistance from connector to module pin. Never guess pin assignments — they vary by vehicle and by connector.
Wiggle-test and heat-cycle the harness while monitoring, for intermittent faults
Tools Scan tool with graphing, heat gun, freeze spray
Work along the harness run near the turbo and charge piping, where heat and vibration do the most damage.
ExpectedA stable signal trace. Any spike that follows your hand or a temperature change localizes the fault.
Substitute a known-good sensor last, then verify with a road test
Tools Known-good sensor, scan tool
Only after wiring, reference, and ground are all proven. Take care removing a sensor from a hot plastic charge pipe — a cracked boss creates a boost leak.
ExpectedSensor reads atmospheric at rest and tracks smoothly with load on a road test. Code does not return through a full drive cycle.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair the shorted or chafed section of the boost sensor signal wiring | Continuity or voltage found between the signal wire and a supply with both ends disconnected, or a fault that appears when a section is flexed | $100–$400 | DIY |
| Clean, dry, or replace the sensor connector and terminals | Oil, moisture, corrosion, or spread terminals found in the connector, with normal readings restored afterward | $100–$300 | DIY |
| Repair the open or high-resistance sensor ground circuit | Excessive voltage drop on the ground leg under load, with the signal returning to a normal range once repaired | $100–$350 | DIY |
| Replace the boost pressure sensor | The reported value dropping to minimum when the signal pin is jumpered to sensor ground with the sensor unplugged, proving wiring and module input good, and correct readings after substituting a known-good sensor | $90–$300 | DIY |
| Repair the circuit or component loading down the shared 5V reference | Reference voltage below nominal that recovers when one specific sensor is disconnected | $100–$400 | DIY |
| Reprogram or replace the engine control module | The value staying pegged high with the signal pin jumpered to sensor ground at the module's own connector, with the signal wire proven continuous and free of shorts to supply | $600–$2,000 | Shop |
US independent shop rates, parts plus labor. The sensor itself is usually inexpensive; the labor depends entirely on where it sits. A sensor on an accessible charge pipe is a ten-minute job, one buried under an intake manifold is not. Diagnostic time for an intermittent wiring fault can exceed the cost of every part involved.
- Buying the sensor first. An open ground and a dead sensor look identical on a scan tool.
- Treating a high reading with the sensor unplugged as proof of a wiring or module fault. These inputs are normally biased high — only jumpering signal to sensor ground separates a bad circuit from a bad sensor.
- Reading this as an overboost code. It says the circuit is out of range, not that the engine overpressured. Fixing the wiring may end the story completely.
- Testing the ground and reference with the connector unplugged, so a resistive fault stays hidden.
- Prying a sensor out of a hot plastic charge pipe and cracking the boss, creating a boost leak and a fresh set of codes.
- Assuming "A" identifies a specific physical sensor on a twin-turbo engine. Which sensor is designated A varies by manufacturer.
- Clearing the code and calling it fixed without driving enough for the monitor to run and the code to have a chance to return.
- Boost Pressure Sensor
- Boost Sensor Wiring
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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