P0238 – Turbo/Super Charger Boost Sensor “A” Circuit High

P0238 means the engine computer sees a boost pressure sensor “A” signal that is too high. Many drivers notice reduced power, poor throttle response, or a sudden limp mode because the computer no longer trusts boost control. This is a circuit high fault, so the problem often comes from wiring, a short to voltage, or a sensor signal stuck high. According to factory diagnostic data, this code indicates the Turbo/Super Charger Boost Sensor “A” circuit reads higher than the allowed range for the current operating conditions. The “A” label is manufacturer-assigned, so confirm which sensor your vehicle calls “A” in service information.

P0238 Quick Answer

P0238 points to a boost/MAP signal circuit that reads too high. Check the boost sensor “A” connector, harness routing, and the 5-volt reference and ground first.

What Does P0238 Mean?

P0238 is defined as “Turbo/Super Charger Boost Sensor ‘A’ Circuit High.” In plain terms, the PCM/ECM detects a boost pressure input that looks unrealistically high. When that happens, the module may limit boost or torque to protect the engine and catalyst. The result can feel like a turbo problem, even when the turbocharger hardware works fine.

Technically, the PCM watches the boost sensor “A” signal voltage and checks it for electrical validity. A “circuit high” result usually means the signal sits higher than expected, often from a short to a voltage source, a sensor internally biased high, or a poor ground that lifts the signal. Diagnosis must prove the circuit fault with measurements before you condemn the sensor or turbo system.

Theory of Operation

The boost sensor (often a MAP sensor on turbo engines, or a dedicated charge-air pressure sensor) converts pressure into a voltage signal. The PCM supplies a regulated reference voltage and a sensor ground. Under normal operation, the signal changes smoothly with key-on engine-off and with load as boost rises and falls.

P0238 sets when the PCM sees the boost sensor “A” circuit return too high for the situation. A short to the 5-volt reference, a rubbed-through harness touching power, or excessive ground resistance can hold the signal high. A skewed signal can also fail plausibility checks against barometric pressure, throttle position, and commanded boost.

Symptoms

P0238 symptoms usually show up first as a drivability change during acceleration or under load.

• Warning light: Check Engine Light on, often after a heavy throttle event or highway merge
• Limp mode: reduced power with limited boost, especially above mid-throttle
• Acceleration: flat power delivery, delayed turbo response, or surging as boost control gets disabled
• Transmission behavior: abnormal shift timing on vehicles that use torque modeling tied to boost
• Fuel economy: noticeable drop if the PCM commands richer fueling or limits efficiency strategies
• Smoke/odor: possible black smoke on some diesels when boost feedback goes irrational
• Scan data: boost/MAP reading appears stuck high or implausible compared to baro at key-on

Common Causes

• Signal circuit short to voltage: A rubbed-through signal wire can contact a power feed and drive the boost sensor “A” signal higher than the PCM expects.
• Boost/MAP sensor “A” internal electrical fault: The sensor can output an abnormally high signal even with normal boost when its internal electronics fail.
• Reference voltage circuit fault (pulled high or cross-fed): A harness cross-connection or backfeed can corrupt the 5V reference and push the sensor output high.
• High-resistance or open sensor ground: A weak ground makes the sensor “float” and read artificially high under vibration or load.
• Connector problems at sensor or PCM: Spread terminals, water intrusion, or pin fit issues can create intermittent high readings that set P0238.
• Harness routing damage near turbo, charge pipe, or EGR components: Heat and movement can melt insulation and intermittently short the circuit high.
• Aftermarket tuning or incorrect sensor scaling: A calibration mismatch can make a normal signal look “too high” to the PCM and trigger P0238.
• Incorrect sensor installed (wrong part number or connector indexing): The wrong sensor can output a different curve and appear high even when the circuit works.
• PCM input circuit issue (rare): An internal PCM pull-up or failed input stage can interpret the signal as high after wiring and sensor checks pass.

Diagnosis Steps

Tools: a scan tool with live data and freeze-frame access, a DVOM, and the correct wiring diagram for your engine. Use back-probes or pin probes to avoid terminal damage. You also need a way to load the circuit during testing. A headlamp bulb load or a fused jumper works well.

1. Confirm the DTC status and record freeze-frame data for P0238. Focus on battery voltage, ignition state, RPM, load, throttle angle, and the boost/MAP PID at the moment it set. Check whether P0238 shows as pending or confirmed. A hard circuit-high fault often returns quickly on key-on.
2. Check fuses and power distribution feeding the boost/MAP sensor circuit and the PCM. Do a fast visual inspection of the sensor circuit path before meter work. Look for melted loom near the turbo, chafing at brackets, and oil-soaked connectors.
3. Verify PCM power and grounds under load with voltage-drop testing. Command loads on if possible, or run the engine and turn on electrical loads. Measure ground drop from the PCM ground pin to battery negative with the circuit operating. Keep ground drop under 0.1V.
4. Inspect the boost sensor “A” connector and terminals closely. Check pin tension, corrosion, water tracks, and pushed-out pins. Wiggle the connector while watching the boost/MAP PID for spikes. Repair terminal fit issues before replacing any parts.
5. Use the scan tool to compare key-on engine-off readings. Boost/MAP should look plausible relative to barometric pressure at rest. If the PID reads abnormally high at KOEO, suspect a circuit high fault rather than an actual boost event.
6. Back-probe the sensor circuits and verify the reference voltage and ground integrity. Do not rely on continuity alone. Load the ground and measure voltage drop from sensor ground to battery negative. A high-resistance ground can pass a continuity test and still cause P0238.
7. Test the signal circuit for short-to-voltage. With the sensor disconnected, check the signal wire for unwanted voltage with ignition on. If you see a strong voltage present, isolate the harness and unplug related connectors to find the backfeed point.
8. Check for signal circuit-to-reference or signal circuit-to-power cross-connection. Inspect harness areas where multiple 5V sensors share a loom. A pinched harness can tie the signal to a 5V feed and force a high signal.
9. If wiring tests pass, perform a sensor functional check using live data. Use a scan tool snapshot during a controlled road test if the fault is intermittent. Freeze frame shows conditions when the code set, while a snapshot captures the moment the concern occurs during diagnosis.
10. Clear codes and confirm the repair. Verify P0238 does not return as pending or confirmed during the same conditions shown in freeze frame. Recheck readiness monitor status after the drive cycle. Remember that clearing codes resets monitors to Not Ready.

Professional tip: When P0238 sets at KOEO or immediately after start, treat it like an electrical fault first. A true overboost event usually tracks with load and throttle. A flat-lined high PID points to a signal circuit short, ground fault, or connector issue.

Possible Fixes

• Repair chafed wiring and correct harness routing near hot or moving components.
• Clean, reseat, or replace damaged terminals at the boost sensor “A” and PCM connectors.
• Restore a solid sensor ground by repairing splices, ground points, or broken ground wires.
• Correct a short-to-voltage or cross-fed reference circuit by isolating and repairing the affected branch.
• Replace the boost/MAP sensor “A” only after circuit tests prove power, ground, and signal wiring integrity.
• Reflash or correct calibration issues when aftermarket tuning or incorrect sensor scaling causes false circuit-high detection.

Can I Still Drive With P0238?

You can often limp the vehicle with a P0238 code, but you should treat it as a “drive carefully and diagnose now” fault. The ECM sets P0238 when it sees the boost pressure sensor “A” signal too high. Many vehicles respond by limiting boost and throttle to protect the engine. Expect reduced power, delayed turbo response, and harsh shifting on some transmissions. Avoid towing, long highway pulls, and hard acceleration until you fix the root cause. If the engine surges, detonates, smokes heavily, or the temperature climbs, stop driving. A falsely high boost signal can also cause incorrect fueling and air control. That can raise exhaust temperatures and stress the turbocharger and catalyst.

How Serious Is This Code?

P0238 ranges from an inconvenience to a real drivability risk, depending on why the signal reads high. When the sensor circuit falsely reads high due to wiring damage, the engine usually runs poorly but stays controllable. You will notice limp mode, reduced boost, and a check engine light. The risk increases if the engine actually overboosts due to a stuck wastegate, boost control solenoid issue, or plumbing problem. True overboost can cause spark knock, misfire under load, and high cylinder pressure. It can also overspeed the turbo and overheat the catalyst. Treat P0238 as high priority if you hear pinging, feel strong surging, or see repeated fuel cut events. Diagnose it before you keep driving hard.

Common Misdiagnoses

Technicians often replace the boost/MAP sensor immediately because the code says “boost sensor.” That wastes money when the real problem sits in the harness. A short to reference voltage, a rubbed-through signal wire touching power, or water in the connector can drive the signal high and set P0238. Another common miss involves sensor identification. The “A” designator does not guarantee the same physical sensor on every platform, so you must confirm which sensor the ECM calls “Boost Sensor A” in service information. Many people also ignore freeze-frame data. That data shows whether the fault occurred at key-on, idle, or under load, which changes the test plan. Finally, some overlook shared 5V reference circuits. A different sensor shorted to voltage can pull the boost signal high and create a false P0238.

Most Likely Fix

The most common confirmed repair direction for P0238 involves correcting the electrical cause of an abnormally high boost sensor “A” signal. Start with connector and harness repairs near the sensor, charge pipe, and intake manifold. Look for oil saturation, pin fit issues, and chafed wiring at brackets. If you verify proper reference voltage and ground, then check the signal wire for a short to voltage and for abnormal pull-up. When the wiring tests clean, validate the sensor by comparing its scan tool reading to a known-good reference, such as BARO/ambient at key-on engine-off. Replace the sensor only after the circuit passes integrity tests. After repairs, road test under boost and confirm the OBD-II monitor completes per enable conditions.

Repair Costs

Repair cost depends on whether the confirmed root cause is a sensor, wiring, connector issue, or control module problem. Verify the fault electrically before replacing parts.

Last updated: April 2, 2026