SPN 723 FMI 8 — Camshaft Speed Abnormal Frequency
SPN 723 FMI 8 means the cam signal frequency is outside the expected range for engine conditions. Noise on the signal wire, a damaged target wheel, and loose sensor mounting are the primary causes — signal integrity and target inspection are required before sensor replacement.
Fault Overview
FMI 8 is the abnormal frequency or out-of-range class on the cam sensor circuit. The ECM monitors cam signal frequency and compares it to expected values for current engine speed — a frequency that does not match the expected pattern sets FMI 8. Noise on the signal wire from poor shielding, a damaged target wheel, loose sensor mounting, and intermittent ground are the primary causes. Poor grounds create noise-like behavior that mimics target damage. A scope is the most effective tool for this fault — DVOM frequency mode can help but may miss intermittent noise events.
Engine & Platform Coverage
Symptoms
- •Cam speed abnormal frequency fault active
- •Cam signal noise indication
- •Sync error from frequency anomaly
- •Intermittent injector disable from frequency fault
- •Possible stall or no-start from sync loss
Possible Causes
- 1.Noise on signal wire — poor shielding or ground integrity
- 2.Damaged target wheel — physical damage to tone features
- 3.Loose sensor mounting — sensor movement creating frequency variation
- 4.Intermittent ground — poor ground creating noise-like behavior
- 5.Shield continuity loss — shield not grounded at one end
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Single Action Gate — Sensor Mount Security Inspection: Inspect cam sensor mounting for looseness. A loose sensor that moves relative to the target wheel produces frequency variation that the ECM interprets as abnormal. Verify mounting torque per OEM specification.
- 2Binary Decision Gate — Tone Ring and Target Inspection: Inspect the reluctor or tone ring for damaged, missing, or deformed features. Physical damage to the target wheel produces a frequency anomaly that cannot be corrected by sensor replacement alone.
- 3Override Manager — Shield and Ground Integrity Check: Verify shield continuity and ground integrity on the cam sensor circuit. Shield continuity and a single-point ground are critical on high-noise engine bays. A broken shield or poor ground creates noise that the ECM interprets as frequency anomalies.
- 4Safety Interceptor — Cam-Crank Correlation PID Comparison: Compare cam and crank correlation PIDs. If cam frequency is abnormal but crank frequency is normal, the fault is isolated to the cam circuit. If both are abnormal, a shared ground or noise source is more likely.
- 5Resume Point — Scope Analysis: Capture cam sensor signal with a scope if available. Look for noise spikes, frequency dropouts, and amplitude variations. A scope provides definitive evidence of noise versus true target damage that DVOM frequency mode may miss.
- 6Verification Loop — Post-Repair Signal Confirmation: After repair, verify cam signal frequency is stable and within expected range. Confirm cam-crank correlation is within OEM specification.
- 7Repair Ledger Output — Sensor, Target, or Shield Decision: Repair shield and ground if noise is confirmed. Replace sensor if mount and target are good and sensor failure is confirmed. Repair or replace target wheel if physical damage is confirmed.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Tighten sensor mounting to OEM specification.
- 2Repair shield continuity and ground integrity.
- 3Repair or replace damaged tone ring or target wheel.
- 4Replace cam sensor after mount, target, and shield integrity are confirmed.
- 5Reroute harness away from noise sources if shielding is inadequate.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
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Related Resources
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Summary
SPN 723 FMI 8 requires signal integrity and target inspection. The AI Diesel Tech FSM enforces the noise versus true mechanical target damage branch before sensor replacement.