SPN 657 FMI 5 — Injector Cylinder 7 Circuit Open
SPN 657 FMI 5 means the ECM detected an open or low-current condition on the cylinder 7 injector driver circuit. Cylinder-local circuit gates stop multi-injector replacement on one open circuit — wiring until the injector proves internally open.
Fault Overview
FMI 5 on the cylinder 7 injector circuit indicates the ECM detected missing or below-threshold current rise during commanded injection. The ECM monitors injector driver current during each injection event and sets FMI 5 when current rise is absent or insufficient. Open connector, broken harness branch, failed injector winding or piezo path, pass-through terminal backout, and oil intrusion bridging then opening intermittently are the primary causes. Service-induced opens after injector replacement when the connector is not fully latched and vibration fatigue at flex loops are documented patterns. Replacing the injector without circuit proof wastes the injector cost when the fault is a disconnected connector or broken harness branch.
Engine & Platform Coverage
Symptoms
- •SPN 657 FMI 5 cylinder 7 injector open
- •Cylinder 7 misfire or no contribution
- •V8 diesel injector wiring fault
- •Rough running under load
- •Increased soot from misfiring cylinder
Possible Causes
- 1.Open connector — connector not fully latched at cylinder 7 injector
- 2.Broken harness branch — wire break in cylinder 7 injector circuit
- 3.Failed injector winding or piezo path — internal injector open
- 4.Pass-through terminal backout — terminal backed out at valve cover pass-through
- 5.Oil intrusion — oil intrusion bridging then opening circuit intermittently
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 — Visual Connector Seat Inspection: Inspect the cylinder 7 injector connector lock and seating. Service-induced opens after injector replacement when the connector is not fully latched are a documented cause. Confirm the connector is fully latched and the lock tab is engaged before any electrical measurements.
- 2Binary Decision Gate — Resistance Check: Measure injector resistance at the cylinder 7 connector. An OL or high resistance reading confirms an open circuit. Compare to OEM injector specification for the specific injector type. 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 7 injector connector to the ECM injector driver connector. A continuity failure confirms a harness break between the injector and the ECM. 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 — Harness vs Injector Isolation: Isolate harness versus injector as the fault source. 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. An internal injector open requires injector replacement with coding or trim where required.
- 6Verification Loop — Post-Repair Contribution Confirmation: After repair, verify cylinder 7 contribution returns to normal. Confirm SPN 657 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 with coding or trim where required 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 7 injector connector.
- 2Repair harness break at valve cover edge, bracket, or pass-through.
- 3Replace injector with coding or trim where required 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 7 open is wiring until the injector proves internally open. The AI Diesel Tech FSM enforces cylinder-local circuit gates to stop multi-injector replacement on one open circuit.