SPN 97 FMI 4 — Water-in-Fuel Sensor Circuit Voltage Below Normal
Low voltage on the WIF circuit is a short-to-ground until the disconnect test proves otherwise.
Fault Overview
FMI 4 sets when voltage on the WIF sensor circuit is below the low threshold, typically a short-to-ground or failed sensor pulling the circuit low.
Engine & Platform Coverage
Symptoms
- •WIF sensor low voltage code
- •Water-in-fuel circuit short
- •False water-in-fuel lamp
- •Continuous WIF indication
Possible Causes
- 1.Signal wire shorted to ground
- 2.Failed sensor internally shorted
- 3.Water intrusion into connector creating low-resistance path
- 4.Damaged insulation near metal brackets
Wiring & Circuit Notes
Look for pinched wires, chafing at clamps, and water-filled connectors. Check routing near the filter housing and frame rails. Harness chafing and connector water intrusion are the most common sources of low-voltage WIF codes.
DVOM Specs & Sensor Tests
Voltage on the signal circuit should not be near zero with a dry sensor. Measure with sensor disconnected — if voltage remains low, the short is in the harness. Sensor resistance should match dry specification.
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 — Measure Signal Voltage with Sensor Connected: Measure signal voltage with sensor connected. Note the reading.
- 2Step 2 — Disconnect Sensor and Remeasure: Disconnect the sensor and remeasure voltage. If voltage rises to near 5 V, the sensor is shorted internally.
- 3Step 3 — Check Sensor Resistance: Measure sensor resistance. Should match dry-bowl specification. Near-zero resistance indicates internal short.
- 4Step 4 — Short-to-Ground Test on Signal Wire: Perform short-to-ground test on the signal wire with sensor disconnected. If voltage remains low, the short is in the harness.
- 5Step 5 — Inspect Harness for Damage: Look for pinched wires, chafing at clamps, and water-filled connectors. Check routing near the filter housing and frame rails.
- 6Step 6 — Repair and Retest: Repair short or replace sensor as isolated. Verify voltage returns to specification after repair.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair shorted wire $50–120; replace shorted sensor $40–120; clean and seal connector $30–80. Total typical $50–200.
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.