SPN 4340 FMI 5 — Aftertreatment DEF Line Heater Circuit Open
SPN 4340 FMI 5 means the DEF line heater circuit is open or drawing below-normal current. Section isolation reduces replacing multiple heated lines at once — identify the open segment before any line replacement.
Fault Overview
FMI 5 on the DEF line heater indicates the circuit is open or drawing below-normal current. Broken heater wire, unplugged inline connector, failed heater element, and open fuse or driver are the primary causes. Lines damaged during tank or aftertreatment R&R and connectors not fully reconnected are documented patterns. Inline connectors along the frame rail are prone to abrasion at clamps and corrosion from DEF leaks. Replacing all heated lines without isolating the open segment wastes parts cost when only one segment is failed.
Engine & Platform Coverage
Symptoms
- •DEF line heater open fault active
- •Frozen DEF line indication
- •Line heater circuit fault
- •No-dose cold weather from frozen line
- •DEF dosing inhibited in cold conditions
Possible Causes
- 1.Broken heater wire — wire break at clamp or abrasion point
- 2.Unplugged inline connector — connector not reconnected after AT or tank R&R
- 3.Failed heater element — open element resistance in line heater
- 4.Open fuse or driver — fuse in line heater circuit
- 5.DEF leak corrosion — urea corrosion at inline connector from DEF leak
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 Audit: Audit all inline connectors along the DEF line heater circuit. Lines damaged during tank or aftertreatment R&R and connectors not fully reconnected are documented causes. Inspect inline connectors along the frame rail for proper engagement and pin condition.
- 2Binary Decision Gate — Resistance Test: Measure line heater circuit resistance versus OEM specification. Infinite resistance or OL indicates an open circuit. Compare resistance across individual line segments to identify the open segment. A reading within specification on one segment with OL on another confirms the open segment location.
- 3Override Manager — Command Voltage Check: Verify command voltage at the line heater circuit when the heater is commanded. No voltage at the line heater feed confirms an open feed circuit or blown fuse. Voltage present with no heater response confirms an open element or circuit downstream of the feed.
- 4Safety Interceptor — Abrasion and DEF Leak Inspection: Inspect inline connectors along the frame rail for abrasion at clamps and corrosion from DEF leaks. DEF leak corrosion at inline connectors is a documented cause of open heater circuits. Clean or replace corroded connectors before any line replacement.
- 5Resume Point — Open Segment Isolation: Isolate the open segment by testing resistance across individual line segments. Replace only the segment with confirmed open resistance. Replacing all heated lines without segment isolation wastes parts cost when only one segment is failed.
- 6Verification Loop — Post-Repair Heater Operation Confirmation: After repair, verify DEF line heater operation in cold ambient conditions. Confirm DEF line temperature rises appropriately with heater active. Monitor for return of FMI 5 over extended cold-weather operation.
- 7Repair Ledger Output — Connector, Segment, or Full Line Decision: Repair connector if found unplugged or corroded. Replace the open line segment if element failure is confirmed. Replace full heated line assembly only if multiple segments are failed or the line is physically damaged.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Reconnect inline connector if found unplugged after AT or tank R&R.
- 2Repair DEF leak corrosion at inline connector.
- 3Replace the open line segment after resistance test confirms the open location.
- 4Replace blown fuse in line heater circuit.
- 5Replace full heated line assembly only if multiple segments are failed or physically damaged.
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
Line heater open is a circuit continuity diagnosis in the heated DEF path. The AI Diesel Tech FSM enforces segment isolation to prevent replacing all heated lines for a single open segment.