P0238 — Turbocharger/Supercharger Boost Sensor A Circuit High — LB7 Duramax
Diagnostic reference for LB7 Duramax (2001–2004) P0238.
What it means
Generic definition: ECM sees boost (MAP) sensor signal voltage above valid range. On LB7 (fixed-geometry turbo) this is a sensor-circuit fault: failed boost sensor or signal wire shorted to 5 V reference.1
Most likely causes
- Boost-pressure (MAP) sensor internal failure causing high output voltage — Sensor drifts high or fails internally, reporting voltage above normal range.1
- Signal wire shorted to 5 V reference or power feed — Boost sensor signal circuit contacts reference/power, driving voltage high.1
- Connector or wiring damage at MAP sensor causing high resistance or short — Corrosion, chafing, or loose terminals alter signal voltage.1
- ECM input fault reading high voltage — ECM internal pull-up or input circuit fault interprets normal signal as high.1
How to diagnose
- Verify code and freeze-frame conditions; confirm P0238 is present and not accompanied by other codes.
- Inspect MAP sensor connector and wiring for corrosion, chafing, or moisture; check harness routing near hot or moving parts.
- Check MAP sensor reference voltage and ground at the connector with key on, engine off; compare to spec.
- Monitor MAP sensor voltage/PID with scan tool at idle and during light load; look for voltage stuck high or out of range.
- Compare scan-tool MAP reading to a mechanical boost gauge to separate real boost from reported boost.
- Inspect intake tract for damage that could affect sensor readings; confirm sensor is properly seated and sealed.
- If voltage remains high with sensor disconnected, suspect wiring short to reference/power or ECM input fault.
- If voltage normal with sensor disconnected but high with sensor connected, suspect sensor internal failure.
- Clear codes, road test, and re-evaluate MAP PID behavior under varying engine load.
- Document findings before replacing components.
Parts and tools you may need
- Digital multimeter
- Scan tool with live data
- Mechanical boost gauge
- Wiring diagram for LB7 MAP sensor circuit
- Contact cleaner
- Basic hand tools for connector access
Tools you’ll need
- OBD-II scan tool — Reads P0238 and live MAP voltage/PID to confirm the sensor signal is stuck high and to compare reported boost vs. actual conditions.
- digital multimeter — Checks MAP sensor 5 V reference, ground, and signal voltage at the connector to identify a short to reference/power or a failed sensor.
- mechanical boost gauge with adapter — Provides an independent boost pressure measurement to separate real intake boost from an erroneous high MAP sensor reading.
- inspection mirror and LED flashlight — Allows visual inspection of the MAP sensor connector and wiring for corrosion, chafing, or moisture in tight engine-bay locations.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| MAP/boost pressure sensor | $40–$200 | 0.3–1 hrs | Easy DIY |
| Engine wiring harness section | $100–$500 | 2–5 hrs | Shop |
| ECM/PCM replacement (programmed) | $600–$2,000 | 1–3 hrs | Shop |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If wiring tests indicate ECM internal fault, or if diagnosis requires probing high-pressure fuel injection systems, consult a qualified diesel technician. Do not perform step-by-step repair of safety-critical systems.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
Spotted an error, or something missing? Tell us
Run a LB7 Duramax? This goes straight to the editor — it is not published.