SPN 729 FMI 6 — Intake Air Heater Control Circuit Current Above Normal
Overcurrent on the intake air heater is a shorted element or load-side short until proven otherwise. Measure resistance and isolated current before replacing any component.
Fault Overview
FMI 6 sets when measured current on the intake air heater circuit exceeds the calibrated high limit. The ECM or smart driver detects overload, short-to-ground on the load side, or a failed element drawing excessive amperage.
Engine & Platform Coverage
Symptoms
- •Heater overcurrent fault active
- •Grid heater short condition
- •Blown fuse on heater circuit
- •Relay clicks then cuts out
- •Cold start aid overload condition
Possible Causes
- 1.Shorted heater element
- 2.Short to ground downstream of the relay
- 3.Incorrect low-resistance aftermarket element installed
- 4.Wiring short after the relay
- 5.Failed relay contacts that do not open cleanly
- 6.ECM current-sense circuit fault (rare)
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 — Disconnect Heater and Measure Resistance: Disconnect the heater element and measure resistance. A shorted element will read near 0 Ω. Compare to OEM specification. A normal element with a wiring short will show normal resistance at the element but abnormal current in the circuit.
- 2Step 2 — Command Circuit with Heater Disconnected: Command the heater circuit on with the element disconnected and confirm current drops to near zero. If current remains high with the element disconnected, the short is in the wiring between the relay and the element connection point.
- 3Step 3 — Inspect Wiring for Short to Ground: Inspect wiring between the relay and heater for contact with grounded surfaces, pinching, or heat damage. Measure resistance from the load wire to chassis ground with the relay disconnected.
- 4Step 4 — Verify Correct Element Part Number and Resistance: Confirm the installed element matches the OEM part number and resistance specification. Aftermarket elements with lower resistance than OEM cause repeated overcurrent faults.
- 5Step 5 — Check Relay for Stuck or Damaged Contacts: Inspect relay contacts for welding or damage. A relay that does not open cleanly can cause sustained overcurrent events.
- 6Step 6 — Confirm Fuse Rating Matches OEM: Verify the installed fuse rating matches OEM specification. An undersized fuse will blow repeatedly; an oversized fuse will not protect the circuit.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Replace shorted element with correct OEM-specification unit.
- 2Repair shorted harness between relay and heater.
- 3Replace damaged relay with correct high-current rating.
- 4Correct any non-OEM heater installation with incorrect resistance.
- 5Replace fuse with correct OEM amperage rating.
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
Overcurrent on the intake air heater is a shorted element or load-side short until proven otherwise. Measure resistance and isolated current before replacing any component.