SPN 626 FMI 3 — Intake Air Heater Relay Circuit Voltage Above Normal
High voltage on the IAH relay circuit is usually a short to power or incorrect relay. Confirm driver type and voltage states before any replacement.
Fault Overview
SPN 626 monitors the intake air heater relay control or feedback circuit. FMI 3 indicates voltage above normal, typically a short to power on the control side or a stuck-closed feedback condition.
Engine & Platform Coverage
Symptoms
- •Heater relay high voltage fault active
- •IAH relay circuit fault
- •Grid heater relay stuck condition
- •Cold start relay code present
Possible Causes
- 1.Control wire shorted to battery voltage
- 2.Relay coil shorted internally
- 3.Incorrect relay installed
- 4.Wiring damage allowing battery feed into control circuit
- 5.ECM driver fault
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 — Identify Driver Type: Determine whether the relay control circuit uses a high-side driver (ECM supplies voltage) or low-side driver (ECM pulls to ground). This determines what "above normal" means for the control circuit and what voltage states are expected.
- 2Step 2 — Measure Control Voltage Key-Off and Key-On/Commanded: Measure voltage at the relay control pin with the relay installed and removed in both key-off and commanded states. Compare to expected values for the identified driver type.
- 3Step 3 — Check for Short to Power on Control Wire: With the relay removed, measure voltage on the control wire. Voltage present when the ECM is not commanding indicates a short to battery voltage in the harness.
- 4Step 4 — Swap with Known-Good Identical Relay: If available, substitute a known-good relay of the identical part number and retest. Confirm the relay is the correct OEM specification for the application.
- 5Step 5 — Verify Power and Ground to Relay Coil and Load Sides: Confirm proper power and ground are present at both the coil and load sides of the relay. Incorrect wiring or a missing ground can create abnormal voltage states.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair shorted control wire with battery voltage feed.
- 2Replace relay with correct OEM-specification unit.
- 3Repair connector with water intrusion or corrosion.
- 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
High voltage on the IAH relay circuit requires driver-type identification and state testing before any module or relay replacement.