SPN 677 FMI 5 — Engine Starter Relay Circuit Open
SPN 677 FMI 5 means the starter relay control circuit is open or not drawing expected current when commanded. The interlock-versus-relay branch prevents unnecessary starter replacement — verify start request and interlocks before any relay or starter work.
Fault Overview
FMI 5 on the starter relay indicates the ECM control circuit is open or not loading the driver as expected. Open control wire, failed relay coil, poor ignition or ECM enable feed, blown fuse, and neutral safety or clutch interlock preventing command are the primary causes. Aftermarket starter interfaces and poor grounds causing intermittent no-crank are documented patterns. The interlock status must be verified before condemning the relay — an interlock preventing the start command produces the same symptom as a failed relay.
Engine & Platform Coverage
Symptoms
- •Starter relay circuit open fault active
- •Click but no crank
- •No-start from relay fault
- •ECM-controlled starter not engaging
- •Intermittent no-crank
Possible Causes
- 1.Open control wire — open between ECM and relay coil
- 2.Failed relay coil — open coil resistance
- 3.Poor ignition or ECM enable feed — no power to relay feed circuit
- 4.Blown fuse — fuse in relay feed circuit
- 5.Neutral safety or clutch interlock — interlock preventing start command
- 6.Aftermarket alarm interrupt — aftermarket device interrupting start circuit
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 — Verify Start Request and Interlocks: Verify the start request is being generated and all interlocks are satisfied. Check neutral safety switch, clutch switch, and any other interlock PIDs with the scan tool. An interlock preventing the start command produces the same symptom as a failed relay. Confirm interlock status before any relay work.
- 2Binary Decision Gate — Check Power at Relay Feed: Verify power at the relay feed circuit during a start request. A blown fuse or open feed circuit prevents the relay from energizing regardless of coil condition. Confirm power is present at the relay feed before measuring the coil control signal.
- 3Override Manager — Measure Coil Control Signal: Measure the coil control signal at the relay during a start request. The ECM should pull the control circuit low to energize the relay. No control signal with confirmed power at the feed confirms an open control wire or ECM output fault.
- 4Safety Interceptor — Relay Test and Replacement: Test the relay coil resistance versus OEM specification. An open coil confirms relay failure. Replace the relay and retest. Confirm the relay base is not damaged from previous overcurrent events before installing a new relay.
- 5Resume Point — Aftermarket Interface Inspection: Inspect for aftermarket starter interfaces and alarm systems that may interrupt the start circuit. Aftermarket devices creating open conditions in the start circuit are a documented cause of intermittent no-crank with relay codes.
- 6Verification Loop — Post-Repair Start Confirmation: After repair, verify the engine cranks normally. Confirm no-crank does not recur. Monitor for return of FMI 5 over extended operation.
- 7Repair Ledger Output — Relay, Wiring, or Interlock Decision: Replace relay if coil failure is confirmed. Repair control wiring if open wire is found. Repair fuse or junction if feed circuit is open. Repair interlock switch if interlock is preventing start command. Proceed to starter cable voltage-drop only after relay path is proven.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Replace relay if coil failure is confirmed.
- 2Repair open control wire between ECM and relay.
- 3Replace blown fuse in relay feed circuit.
- 4Repair interlock switch if interlock is preventing start command.
- 5Repair aftermarket interface interrupting start circuit.
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
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
Starter relay open codes are control-side electrical diagnostics first. The AI Diesel Tech FSM enforces the interlock-versus-relay branch to prevent unnecessary starter replacement.