SPN 626 FMI 4 — Intake Air Heater Relay Circuit Voltage Below Normal
Low voltage on the IAH relay control circuit points to a short to ground or open until wiring is proven good.
Fault Overview
FMI 4 indicates voltage below normal on the intake air heater relay control circuit, commonly a short to ground or open that pulls the circuit low when it should be high (or vice versa depending on driver type).
Engine & Platform Coverage
Symptoms
- •IAH relay low voltage fault active
- •Heater relay circuit short condition
- •Grid heater relay not energizing
Possible Causes
- 1.Control wire shorted to ground
- 2.Open control wire
- 3.Failed relay coil
- 4.Poor ground on low-side driver circuit
- 5.ECM driver failed low
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 — Measure Control Voltage in Both Commanded States: Measure voltage at the relay control pin in both the off and commanded-on states. Compare to expected values for the driver type. Low voltage in the commanded-on state confirms the circuit is not reaching the expected level.
- 2Step 2 — Check Coil Resistance: Measure relay coil resistance. Typical range is 50–120 Ω depending on relay specification. An open coil reads infinite resistance.
- 3Step 3 — Continuity and Short-to-Ground Tests on Control Wire: With the relay removed, measure resistance from the control wire to chassis ground. A short to ground will read near 0 Ω. Perform continuity test end-to-end on the control wire.
- 4Step 4 — Verify Power Feed to Relay: Confirm battery voltage is present at the relay power feed terminal. A missing power feed will prevent the relay from operating regardless of control circuit condition.
- 5Step 5 — Load-Test the Circuit: If available, load-test the control circuit to confirm it can sustain the required current without dropping below threshold.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair shorted or open control wire.
- 2Replace relay with failed coil.
- 3Clean corroded relay socket.
- 4Repair ECM driver circuit if confirmed as source after wiring is proven good.
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
Low voltage on the IAH relay control circuit points to a short to ground or open until wiring is proven good.