P0238 — Turbocharger/Supercharger Boost Sensor A Circuit High — 5.9 Cummins 24-Valve (VP44)
Diagnostic reference for P0238 on 5.9L Cummins 24-Valve VP44 (1998.5-2002).
What it means
Generic definition: ECM sees boost (MAP) sensor A signal voltage above its valid range. On the 5.9L 24-valve the turbo is a fixed-geometry Holset HX35/HY35, so this is a sensor-circuit fault: a failed boost sensor, a signal wire shorted to the 5 V reference, or a connector/wiring fault.1
Most likely causes
- Boost/MAP sensor internal failure causing high output voltage — Sensor drifts high or fails internally, driving signal above valid range.1
- Signal wire shorted to 5 V reference or power — Chafed harness or pinched wire pulls sensor signal up to reference voltage.1
- Connector corrosion/moisture or poor terminal contact at boost sensor — High resistance or intermittent contact can bias the signal high.1
- ECM 5 V reference circuit fault affecting sensor supply — Reference voltage issue can raise sensor output reading.1
How to diagnose
- Verify code and freeze frame; confirm P0238 is present and note engine conditions at set.
- Inspect boost/MAP sensor and wiring for damage, oil contamination, chafing, and connector corrosion.
- Check sensor reference voltage and ground at the sensor connector with key on, engine off; compare to spec.
- Backprobe sensor signal wire and monitor voltage at idle and with RPM increase; look for voltage stuck high or out of range.
- Perform wiggle test on harness and connector while watching live data for signal jumps.
- Inspect ECM connector and harness routing for chafing near valve cover, intake, and firewall.
- If wiring and reference are normal, substitute a known-good boost/MAP sensor and re-evaluate.
- Clear codes and road test to confirm repair.
Parts and tools you may need
- Digital multimeter
- Scan tool with live data
- Backprobe pins
- Dielectric grease
- Replacement boost/MAP sensor
Tools you’ll need
- digital multimeter — Measures reference voltage, ground, and signal voltage at the MAP sensor connector to confirm a stuck-high or out-of-range signal.
- OBD-II scan tool — Reads P0238, freeze frame, and live MAP/boost data to verify the fault and monitor signal changes during wiggle tests.
- backprobe kit — Allows safe access to the MAP sensor signal, 5 V reference, and ground wires while the connector remains plugged in.
- inspection mirror — Helps view the MAP sensor and harness routing in tight areas near the intake and firewall for chafing or corrosion.
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 |
| 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 faults are found in the ECM harness or intermittent high readings persist after sensor replacement, have a qualified diesel technician diagnose the ECM reference circuit and harness integrity. Do not attempt high-pressure fuel system service. If the vehicle shows drivability issues, have it inspected promptly.
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 5.9 Cummins 24-Valve (VP44)? This goes straight to the editor — it is not published.