SPN 3515 FMI 4 — Aftertreatment DEF Dosing Valve Circuit Voltage Below Normal
DEF dosing valve low-voltage codes require short isolation before valve replacement. Repeat DEF dosing valve failures from unresolved shorted control wires are the most documented pattern.
Fault Overview
SPN 3515 FMI 4 indicates the aftertreatment DEF dosing valve circuit voltage is below the normal minimum — a short circuit or low-signal class fault. Control wire short to ground, failed valve with internal short, crushed harness, and moisture bridging are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Control wire short to ground at pinch points near DEF dosing valve
- 2.Failed DEF dosing valve with internal short to ground
- 3.Crushed harness from improper routing during DEF system service
- 4.Moisture bridging at connector body near DEF dosing valve
- 5.Melted insulation near heat sources bridging control wire to ground
Wiring & Circuit Notes
Inspect pinch points and melt locations near the DEF dosing valve — these are the highest-probability short locations for the valve control circuit. Check for moisture in the connector body.
DVOM Specs & Sensor Tests
Disconnect the dosing valve and check the control wire for short to ground. If the short clears with the valve disconnected, the valve is internally shorted. If the short persists, inspect the harness at pinch and melt points. Replace the valve only if the circuit is clean — installing a new valve into a shorted harness will damage the replacement.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Disconnect DEF dosing valve connector
- 2Check control wire for short to ground with valve disconnected
- 3If short clears with valve disconnected, replace valve
- 4If short persists, inspect harness at DEF dosing valve pinch and melt points
- 5Repair harness short at identified location
- 6Replace valve only after circuit is confirmed clean
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Disconnect DEF dosing valve connector
- 2Check control wire for short to ground with valve disconnected
- 3If short clears with valve disconnected, replace valve
- 4If short persists, inspect harness at DEF dosing valve pinch and melt points
- 5Repair harness short at identified location
- 6Replace valve only after circuit is confirmed clean
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
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Summary
SPN 3515 FMI 4 requires short isolation before DEF dosing valve replacement. An unresolved harness short will damage replacement valves.