SPN 5397 FMI 31 — Aftertreatment DEF Pump Pressure Condition
SPN 5397 FMI 31 means the DEF pump pressure is outside expected operation. This is a hydraulic and electrical diagnosis — restriction, freeze damage, and line leaks are more common than pump module failure. Prove the hydraulic path and electrical supply before replacing the pump module.
Fault Overview
FMI 31 is a condition-exists class that indicates the DEF pump pressure is outside the expected operating range for the platform. The DEF pump module pressurizes the DEF system for dosing. Failed pump, restricted filter or lines, frozen DEF, leaking lines, pressure sensor faults, and electrical pump control issues are all causes. Winter freeze damage and contamination plugging the pump filter are the most common fleet patterns. Pump module replacement without proving the hydraulic path and electrical supply is the primary misdiagnosis.
Engine & Platform Coverage
Symptoms
- •DEF pump pressure fault active
- •DEF system pressure low or high
- •Dosing pressure code
- •SCR efficiency declining from dosing failure
- •Inducement risk from dosing loss
Possible Causes
- 1.Failed pump — internal pump failure
- 2.Restricted filter — contamination plugging pump inlet filter
- 3.Frozen DEF — winter freeze damage to pump or lines
- 4.Leaking lines — pressure loss from line damage
- 5.Pressure sensor fault — false pressure reading
- 6.Electrical pump control fault — supply or control circuit issue
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Single Action Gate — DEF Level and Quality Check: Verify DEF level and quality before any pump diagnosis. Low DEF level causes pump cavitation that mimics pump failure. Contaminated DEF can plug the pump filter rapidly.
- 2Binary Decision Gate — Freeze and Restriction Inspection: Inspect DEF lines and pump module for freeze damage and restriction. In cold climates, winter freeze damage is the most common cause of DEF pump pressure faults. Inspect lines for ice blockage and pump module for freeze-crack damage.
- 3Override Manager — Pump Command Test: Command pump operation via scan tool and observe pressure response. Compare commanded pump duty to reported pressure. No pressure buildup with valid electrical supply supports pump or hydraulic restriction failure.
- 4Safety Interceptor — Line Leak Inspection: Inspect all DEF lines for leaks. A leaking pressure line prevents pressure buildup regardless of pump condition. Inspect fittings, connections, and the dosing injector for DEF leaks.
- 5Resume Point — Pump Module Electrical Supply: Verify pump module supply voltage and ground under operation. Pump module connector corrosion from DEF crystals is a documented cause. Inspect connector for urea crystal buildup and corrosion.
- 6Verification Loop — Post-Service Pressure Confirmation: After repair, command pump and verify pressure builds to OEM specification. Confirm dosing quantity returns to expected levels.
- 7Repair Ledger Output — Pump Module Replacement: Replace pump module only after hydraulic path and electrical supply are proven. Note: some platforms require DEF system purge and prime after pump replacement.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Thaw frozen DEF lines and inspect for freeze-crack damage.
- 2Clean or replace pump inlet filter if restriction is found.
- 3Repair any DEF line leaks found.
- 4Repair pump module connector corrosion from DEF crystal buildup.
- 5Repair electrical supply faults if found.
- 6Replace pump module after hydraulic and electrical proof.
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
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
SPN 5397 FMI 31 requires hydraulic and electrical proof before pump module replacement. The AI Diesel Tech FSM enforces restriction and freeze gates before the pump replacement branch opens.