SPN 658 FMI 5 — Injector Cylinder 8 Circuit Open
SPN 658 FMI 5 means the ECM detected an open or low-current condition on the cylinder 8 injector driver circuit. Cylinder 8 rear-bank connectors are often disturbed during turbo or aftertreatment service — connector re-lock verification is the mandatory first step.
Fault Overview
FMI 5 on the cylinder 8 injector circuit indicates the ECM detected missing or below-threshold current rise during commanded injection. Unseated connector, open wire, failed injector, damaged pass-through, and corrosion are the primary causes. Repeat opens after incomplete connector engagement under valve covers are a documented pattern on rear-bank cylinders. Harder-access rear-bank connectors are often disturbed during turbo or AT service on some engine packages — a disconnected connector after service is the most efficient first check.
Engine & Platform Coverage
Symptoms
- •SPN 658 FMI 5 cylinder 8 injector open
- •Cylinder 8 dead miss or no contribution
- •V8 injector circuit fault
- •Rough running under load
- •Increased soot from misfiring cylinder
Possible Causes
- 1.Unseated connector — connector not fully engaged at cylinder 8 injector
- 2.Open wire — wire break in cylinder 8 injector circuit
- 3.Failed injector — internal injector open
- 4.Damaged pass-through — pass-through terminal damage at valve cover
- 5.Corrosion — connector corrosion from oil or moisture intrusion
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 — Rear-Bank Connector Access and Lock Confirmation: Access the cylinder 8 rear-bank connector and confirm lock engagement. Harder-access rear-bank connectors are often disturbed during turbo or AT service. Confirm the connector is fully latched and the lock tab is engaged before any electrical measurements. A disconnected connector after service is the most efficient first check.
- 2Binary Decision Gate — Resistance Check: Measure injector resistance at the cylinder 8 connector. An OL or high resistance reading confirms an open circuit. Compare to OEM injector specification. A resistance within specification with a confirmed open circuit code confirms a harness fault between the connector and the ECM.
- 3Override Manager — Continuity to Controller: Verify continuity from the cylinder 8 injector connector to the ECM injector driver connector. A continuity failure confirms a harness break. Inspect harness near valve cover edges, brackets, and pass-through terminals for the break location.
- 4Safety Interceptor — Verify No Short Before Re-Actuation: Verify no short-to-ground or short-to-power exists before re-actuating the injector driver. A short in the circuit can damage the ECM injector driver. Confirm circuit integrity before any injector actuation tests.
- 5Resume Point — Component Decision After Circuit Proof: Proceed to injector replacement only after circuit integrity is confirmed and internal injector open is proven. A harness with confirmed continuity and a connector with confirmed seating that still shows OL resistance at the injector pins confirms an internal injector open.
- 6Verification Loop — Post-Repair Contribution Confirmation: After repair, verify cylinder 8 contribution returns to normal. Confirm SPN 658 FMI 5 does not return. Monitor for return of misfire or rough running under load.
- 7Repair Ledger Output — Connector, Harness, or Injector Decision: Re-seat or repair connector if disconnected or damaged. Repair harness break if found. Replace injector only after internal open is confirmed. Do not replace injector for a connector or harness fault.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
1 steps locked- 1Re-seat or repair cylinder 8 injector connector.
- 2Repair harness break at valve cover edge, bracket, or pass-through.
- 3Replace injector only after internal open is confirmed.
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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Summary
Cylinder 8 open follows the same circuit-to-component sequence as other injector open codes. The AI Diesel Tech FSM prevents bank-wide injector replacement for one circuit fault.