SPN 626 FMI 5 — Intake Air Heater Relay Circuit Open
Open circuit on the IAH relay is a wiring, fuse, or coil problem until proven otherwise. Prove continuity before replacing any module.
Fault Overview
FMI 5 indicates an open circuit in the intake air heater relay control or load path as detected by the ECM current or voltage monitoring.
Engine & Platform Coverage
Symptoms
- •IAH relay open circuit fault active
- •Heater relay no continuity
- •Grid heater relay open condition
Possible Causes
- 1.Open control wire
- 2.Open relay coil
- 3.Unplugged connector
- 4.Blown fuse on control side
- 5.Failed open driver
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Check Control-Side Fuse: Inspect all fuses in the relay control circuit. A blown control-side fuse will prevent the relay from receiving the command signal.
- 2Step 2 — Measure Coil Resistance: Measure relay coil resistance. An open coil reads infinite resistance and confirms relay failure.
- 3Step 3 — Continuity Test Control Wire: Perform end-to-end continuity test on the control wire from ECM to relay socket. An open wire reads infinite resistance.
- 4Step 4 — Verify Power and Ground at Relay: Confirm battery voltage is present at the relay power feed terminal and ground is solid at the ground terminal.
- 5Step 5 — Command and Monitor Response: Command the relay and monitor for energization. If power, ground, and control wire are all good but the relay does not energize, confirm coil resistance and substitute a known-good relay.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Replace open relay with failed coil.
- 2Repair open control wire.
- 3Replace blown control-side fuse and identify root cause.
- 4Restore connector with backed-out or corroded pins.
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
Open circuit on the IAH relay is a wiring, fuse, or coil problem until proven otherwise. Prove continuity before replacing any module.