SPN 975 FMI 5 — Engine Fan Drive Circuit Open / Current Below Normal
Fan open-load codes are a locate-the-open process, not automatic fan replacement. Fan commanded on in software with no current because the connector was left loose after condenser or fan service is the most documented cause of this fault class.
Fault Overview
SPN 975 FMI 5 indicates the fan drive circuit has an open load — current is below normal for the commanded state. Open coil, disconnected connector, broken feed wire, failed driver open, and high-resistance connections acting like an open under load are the primary causes. Voltage-versus-current analysis is required to locate the open before any fan drive replacement.
Engine & Platform Coverage
Symptoms
- •Fan commanded on with no physical engagement
- •Voltage present at fan connector but no current flow
- •Fan connector left loose after cooling service
- •SPN 975 FMI 5 active after fan or condenser service
- •Open-load fault with no visible harness damage
Possible Causes
- 1.Open coil in the fan drive assembly
- 2.Disconnected connector at the fan drive — left loose after service
- 3.Broken feed wire in the fan control circuit
- 4.Failed driver open in the ECM or relay
- 5.High-resistance connection acting like an open under load
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 — Confirm Fan Command: Verify the ECM is commanding the fan on. Check the fan request PID under conditions that should trigger fan engagement. If the ECM is not commanding the fan, diagnose the input sensors before the fan circuit.
- 2Step 2 — Voltage at Fan Under Command: With the fan commanded on, measure voltage at the fan drive connector. Voltage present with no current flow supports an open load in the fan drive or a disconnected connector.
- 3Step 3 — Current or Resistance Check: Measure current in the fan control circuit under command if the process supports it. Measure coil or actuator resistance at the fan drive. An open coil reads infinite resistance.
- 4Step 4 — Isolate the Open Section: Trace the open from the fan drive back toward the ECM or relay. Voltage present at the fan connector with an open coil confirms the fan drive is the fault. Voltage absent at the connector confirms an upstream open in the harness or relay.
- 5Step 5 — Replace Fan Drive Only If Internal Open Is Proven: Replace the fan drive assembly only after the internal open is confirmed. If the open is in the harness or relay, repair the circuit first.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Reconnect loose fan connector left after service.
- 2Repair broken feed wire in the fan control circuit.
- 3Replace failed relay with open output.
- 4Replace fan drive assembly after internal open coil is confirmed.
- 5Repair high-resistance connection at fan circuit connector.
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
SPN 975 FMI 5 is a fan circuit open-load fault. Locate the open — connector, harness, relay, or fan drive coil — before any replacement.