P1138 — Diesel pressure circuit fault

Diagnostic algorithm

1. Retrieve DTCs and freeze-frame data; note operating conditions when fault set (engine speed, temperature, command pressure).
2. Perform a visual inspection of the rail pressure sensor, connector and harness back to the ECM for damage, corrosion, or water ingress.
3. With a digital multimeter, verify sensor supply (reference) voltage and ground at the sensor connector with ignition ON. Replace/repair fuse or ground if absent or out of range.
4. Measure sensor output voltage at ignition ON and while cranking/running. Compare to OEM expected voltages or behavior. Look for stuck, pulsing or out-of-range values.
5. Wiggle test wiring and connector while monitoring live data for intermittent faults. Repair any wiring faults found.
6. Install an independent high-pressure fuel rail gauge to verify actual rail pressure under various engine loads and compare to sensor reading on the scan tool.
7. If sensor output does not match actual pressure and wiring/power/ground are good, replace the fuel rail pressure sensor and retest.
8. If pressure sensor and wiring test good but pressure is out of expected range, inspect/repair high-pressure pump, pressure control valve (regulator), filters and fuel supply components.
9. After repairs, clear codes and perform road test under conditions that previously set the code; confirm no reoccurrence and verify live data matches expected values.
10. If problems persist, consult factory wiring diagrams and perform module-level diagnostics (ECM input/output checks) or seek manufacturer technical service information.