SPN 38 FMI 1 — Secondary Fuel Level Below Normal (Dual Tank Systems)
SPN 38 FMI 1 means the secondary tank fuel level is below normal on dual-tank configurations. Secondary low-level diagnosis must include transfer system function — transfer pump failures are misdiagnosed as primary supply pump issues when the transfer system is not evaluated.
Fault Overview
FMI 1 on secondary fuel level indicates the secondary tank level is below the normal threshold. True low secondary tank, failed secondary sender, transfer pump failure, clogged transfer lines or valves, and control switch faults are the primary causes. Transfer pump failures misdiagnosed as primary supply pump issues are a documented pattern on dual-tank systems. Dual-tank logic prevents wrong-tank and wrong-pump repairs by evaluating the transfer system before any pump diagnosis.
Engine & Platform Coverage
Symptoms
- •SPN 38 FMI 1 secondary fuel level low
- •Right tank empty or low
- •Dual tank level fault
- •Transfer system not working
- •Secondary fuel level low warning
Possible Causes
- 1.True low secondary tank — secondary tank genuinely low from consumption or transfer failure
- 2.Failed secondary sender — secondary fuel level sender failure
- 3.Transfer pump failure — transfer pump not moving fuel from secondary to primary
- 4.Clogged transfer lines or valves — restriction in transfer path
- 5.Control switch faults — transfer control switch fault preventing transfer
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 — Physical Levels in Both Tanks: Verify physical fuel levels in both the primary and secondary tanks. Physical level proof in both tanks identifies whether the secondary tank is genuinely low or the sender is reading incorrectly. A secondary tank that is confirmed full with a low reading confirms a sender or wiring fault.
- 2Binary Decision Gate — Command and Observe Transfer: Command the transfer system and observe transfer operation. A transfer system that does not move fuel when commanded confirms a transfer pump, line, or valve fault. A transfer system that moves fuel confirms the transfer path is functional.
- 3Override Manager — Voltage at Transfer Pump: Verify voltage at the transfer pump when commanded. No voltage at the transfer pump confirms a control circuit fault — switch, relay, or wiring. Voltage present with no transfer pump operation confirms a failed transfer pump.
- 4Safety Interceptor — Line Restriction Checks: Inspect transfer lines and valves for restriction or blockage. A clogged transfer line or valve prevents fuel transfer even when the pump is operating. Inspect transfer lines, check valves, and selector valves for restriction.
- 5Resume Point — Secondary Sender Tests: Perform secondary sender tests if the physical level and transfer system are confirmed normal but the level reading remains low. Inspect the secondary sender connector and ground integrity. Perform sender resistance sweep where applicable.
- 6Verification Loop — Post-Repair Level and Transfer Confirmation: After repair, verify secondary tank level reads correctly and transfer system operates normally. Confirm SPN 38 FMI 1 does not return. Monitor for return of secondary level faults over extended operation.
- 7Repair Ledger Output — Refuel, Transfer System, or Sender Decision: Refuel secondary tank if true low volume is confirmed. Repair transfer pump, lines, or valve if transfer fault is confirmed. Repair control circuit if switch or relay fault is confirmed. Replace secondary sender if failure is confirmed after circuit proof.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Refuel secondary tank if true low volume is confirmed.
- 2Replace transfer pump if pump failure is confirmed.
- 3Repair transfer lines or valve if restriction is confirmed.
- 4Repair control circuit if switch or relay fault is confirmed.
- 5Replace secondary sender if failure is confirmed after circuit 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
Secondary low-level diagnosis must include transfer system function. The AI Diesel Tech FSM enforces dual-tank logic to prevent wrong-tank and wrong-pump repairs.