Detailed guide for SPN 108 FMI 18
Back to top ↑Barometric pressure data is used to adjust fueling and boost, especially in mountainous regions.
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Share ↓SPN 108 FMI 18Severity: Warning
SPN 108 FMI 18: Barometric pressure sensor below normal – altitude or sensor issue
Quick Answer
When SPN 108 FMI 18 sets, the ECM is indicating a fault condition involving the component or system named in the code.
OEM definitions and thresholds can vary, so confirm the exact meaning with service information or a capable diagnostic tool.
Code
SPN 108 FMI 18
Severity
Warning
Applies to
Check OEM application
Can I drive?
A short trip may be possible if the truck is operating normally and there is no stop-engine or critical warning. Diagnose SPN 108 FMI 18 soon and use the live-data and circuit checks below to determine how urgent the fault is.
Common Symptoms
- Reduced power at altitude
- Fuel economy changes
Likely Causes
- Sensor drift
- Blocked sensor vent
Quick Diagnostic Checks
Check first- 1Scan for companion codes and record freeze-frame data (if available).
- 2Inspect connectors, harness rub points, and pins for corrosion.
- 3Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).
- 4After repair, clear codes and confirm the fault stays inactive after a drive cycle.
Common Fixes
- Inspect sensor vent
- Replace sensor if readings are inaccurate
Full technical sections↓
What does SPN 108 FMI 18 mean?
Back to top ↑When SPN 108 FMI 18 sets, the ECM is indicating a fault condition involving the component or system named in the code.
OEM definitions and thresholds can vary, so confirm the exact meaning with service information or a capable diagnostic tool.
Treat repeat faults as ACTIVE and diagnose using live data, wiring checks, and basic inspections before replacing parts.
How this system works (why SPN 108 FMI 18 happens)
Back to top ↑Most modern fault logic is based on expected versus actual sensor readings, timing windows, circuit checks, and plausibility tests.
A stored code can come from a temporary event, while a repeatable active code gives you a much better diagnostic target.
The goal is to identify whether the failure is electrical, mechanical, sensor-related, or caused by another upstream fault before replacing components.
Location hint for SPN 108 FMI 18
Back to top ↑Component location varies by OEM and chassis. Use your engine’s service manual to confirm the sensor/valve location before replacing parts.
Tip: Inspect connectors and harness routing before replacing parts.
Scan tool checks for SPN 108 FMI 18
Back to top ↑- Check status: ACTIVE vs INACTIVE / stored and review freeze-frame data when available.
- Record companion codes and diagnose the earliest or most upstream fault first.
- Compare live sensor values with expected operating conditions and inspect wiring and connectors.
- Clear and re-test under similar conditions only after completing the initial inspection.
If your tool can’t run actuator tests, you can still diagnose a lot with careful inspection + repeatability testing.
Step-by-step diagnostic for SPN 108 FMI 18
Back to top ↑- Visual inspection (fast wins). Harness routing, connector pins, hoses/lines (if applicable), and obvious damage.
- Check fluid sanity. Not applicable unless your fault is fluid-related. Focus on inspections + repeatability.
- Confirm the fault is ACTIVE. Stored-only faults can be old events.
- Review live data and circuit values. Use OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) or a capable heavy-duty scan tool to review the affected signal, commanded/actual values when applicable, and related sensor data.
- Verify after repair. Clear codes, road test, confirm it stays inactive.
Decision guide (fast path) for SPN 108 FMI 18
Back to top ↑- If the fault is STORED only → inspect, clear, and monitor. If it returns, treat it as active.
- If ACTIVE → inspect the named component, connector, harness, supply/ground, and live-data plausibility.
- If multiple codes are present → diagnose the earliest or most upstream code first.
Common misdiagnosis mistakes for SPN 108 FMI 18
Back to top ↑- Replacing sensors or components without checking wiring, grounds, connector fitment, and companion codes.
- Clearing the fault repeatedly without confirming whether it is ACTIVE and repeatable.
- Missing an upstream fault that is causing the displayed code as a secondary symptom.
Will SPN 108 FMI 18 cause derate or inducement?
Back to top ↑A warning fault may allow continued operation, but the effect depends on the failed component and any companion faults.
Exact behavior depends on ECM calibration and which companion faults are active.
“Do this first” checklist for SPN 108 FMI 18
Back to top ↑- Scan for companion codes and record freeze-frame data (if available).
- Inspect connectors, harness rub points, and pins for corrosion.
- Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).
- After repair, clear codes and confirm the fault stays inactive after a drive cycle.
Tools needed to diagnose SPN 108 FMI 18
Back to top ↑- Basic hand tools + good lighting for inspection
- Multimeter for connector/power/ground checks (if wiring suspected)
- OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) (or equivalent scan tool) for live data and supported diagnostic tests
Prevention tips to avoid repeat SPN 108 FMI 18
Back to top ↑- Fix small wiring/connector issues early before they become repeat faults.
- During service, inspect harness routing and clamp points near heat/vibration.
- Don’t ignore intermittent faults. “Erratic” usually gets worse.
What to check first for SPN 108 FMI 18
- Inspect connectors and harness near the affected component for corrosion, rubbing, or loose pins.
- Check for upstream causes (communications, sensors, air/fuel issues).
- Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).
If the fault repeats after clearing, diagnose it as ACTIVE and review companion codes and live data before replacing parts.
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