SPN 729 FMI 5 — Intake Air Heater Control Circuit Open / Current Below Normal
FMI 5 on the intake air heater is an open or low-current condition. Confirm with resistance and actual current draw under command before replacing any component.
Fault Overview
FMI 5 indicates the ECM detects current below the expected minimum or an open-circuit condition on the intake air heater control or load circuit. Monitoring may use current sense in the driver or voltage feedback that never leaves the open-circuit state when commanded.
Engine & Platform Coverage
Symptoms
- •Grid heater open circuit
- •No current on cold start aid
- •Hard start in cold weather
- •No preheat before cranking
- •White smoke cold only
Possible Causes
- 1.Open heater element
- 2.Broken power or ground cable
- 3.Unplugged or corroded connector
- 4.Failed open relay
- 5.Blown high-amperage fuse
- 6.Open control wire from ECM to relay
- 7.High-resistance connection limiting current below threshold
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Measure Element Resistance Cold: Measure element resistance at the studs. Normal range is low (0.2–1.0 Ω platform specific). Open circuit reads infinite resistance and confirms element failure.
- 2Step 2 — Command Heater and Confirm Current Draw: Command heater on and measure current draw with an inductive amp clamp. Expected current is platform-specific but typically tens of amps. No current with element connected confirms an open in the circuit path.
- 3Step 3 — Check Continuity Through Power and Ground Paths: Verify continuity through the entire positive feed and ground return path. Locate the open by measuring voltage at each connection point in sequence.
- 4Step 4 — Verify Relay Coil and Contact Operation: Confirm relay coil resistance within specification. Verify relay contacts close when coil is energized. An open relay contact will prevent current from reaching the element.
- 5Step 5 — Inspect and Clean High-Current Terminals: Inspect all high-current terminals for corrosion, looseness, or heat discoloration. Wiggle-test connectors while monitoring current or voltage for intermittent opens.
- 6Step 6 — Confirm ECM Control Signal Reaches Relay: Verify ECM control signal is present at the relay coil. If the relay is not being commanded, diagnose the enable conditions before the circuit.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Replace open element confirmed by infinite resistance.
- 2Repair or replace open power or ground cables.
- 3Replace failed open relay.
- 4Restore connector integrity at corroded or backed-out terminals.
- 5Replace blown high-amperage fuse and identify root cause.
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
FMI 5 on the intake air heater is an open or low-current condition. Confirm with resistance and actual current draw under command before replacing any component.