Diesel Engines
How to read SPN and FMI fault codes
SPN tells you what circuit or component the ECM is unhappy about. FMI tells you how it is unhappy. Together they point you at a wiring test, not a parts order.
The answer
The two halves
- SPN — Suspect Parameter Number. The component or data point, for example the intake manifold pressure sensor.
- FMI — Failure Mode Identifier. The nature of the failure: shorted high, shorted low, data erratic, out of range, not responding.
The common FMIs and what they actually mean
- FMI 3 — voltage above normal or shorted high. Usually a chafed wire touching power, or an open ground.
- FMI 4 — voltage below normal or shorted low. Usually a wire shorted to chassis or a failed sensor.
- FMI 5 — current below normal or open circuit. Look for a broken wire or unplugged connector.
- FMI 2 — data erratic or intermittent. Wiggle-test the harness with live data on screen.
- FMI 9 or 19 — abnormal update rate or network error. This is a datalink or module problem, not the sensor.
The habit that saves money
Before replacing any sensor, unplug it and read live data. A shorted-low reading that jumps to full scale when unplugged proves the sensor, not the harness. If nothing changes, you have a wiring problem.