SPN 105 FMI 0 — Intake Manifold Temperature Above Normal
SPN 105 FMI 0 means the intake manifold or air temperature is above the calibrated limit. This is a thermal-management and sensing diagnosis — distinguish true CAC efficiency loss from sensor bias before any component replacement.
Fault Overview
FMI 0 on intake manifold temperature indicates the sensor reports above the maximum calibrated limit. Heat-soaked sensor, failed CAC efficiency, recirculation of hot air, sensor bias high, and extreme ambient combined with high load are the primary causes. CAC efficiency loss from internal restriction or external debris packing is a documented cause that produces true high intake temperatures. Ignoring high IAT faults as weather-related nuisance codes is a documented pattern that leads to power loss and derate under hot ambient conditions.
Engine & Platform Coverage
Symptoms
- •Intake air temperature high fault active
- •Hot CAC outlet indication
- •Derate from high IAT
- •Heat-soaked intake indication
- •Power loss under high ambient and load conditions
Possible Causes
- 1.Heat-soaked sensor — sensor reading above actual air temperature
- 2.Failed CAC efficiency — internal restriction or external debris pack
- 3.Recirculation of hot air — missing air seals recirculating engine heat
- 4.Sensor bias high — thermistor reading above actual temperature
- 5.Extreme ambient plus high load — thermal accumulation in intake system
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 — Sensor Correlation Check: Compare IAT scan data to an infrared measurement at the sensor boss. A large disagreement between scan data and infrared measurement supports sensor bias. A reading that correlates with infrared measurement supports true high intake temperature.
- 2Binary Decision Gate — CAC Airflow Package Inspection: Inspect the charge air cooler airflow package for external debris packing and internal restriction. Vocational trucks and trucks operating in debris-heavy environments are prone to packed coolers that reduce CAC efficiency.
- 3Override Manager — Duct and Seal Inspection: Inspect intake ducting for missing or damaged air seals that allow hot engine bay air to recirculate into the intake. Missing seals between the CAC and the intake manifold are a documented cause of elevated intake temperatures.
- 4Safety Interceptor — Sensor Circuit Verification: Verify sensor circuit integrity if sensor correlation check suggests bias. IAT sensors are typically thermistor style — measure resistance and compare to OEM temperature-resistance curve. Inspect connector for heat damage near intake plumbing.
- 5Resume Point — Hard-Pull Condition Retest: If the fault is intermittent, retest under hard-pull conditions. True CAC efficiency loss produces elevated intake temperatures under high load that may not be apparent at idle.
- 6Verification Loop — Post-Repair Temperature Confirmation: After repair, verify intake temperature remains within OEM specification under high ambient and high load conditions. Confirm derate does not activate after repair.
- 7Repair Ledger Output — CAC, Seal, or Sensor Decision: Clean or replace CAC if efficiency loss is confirmed. Repair or replace missing air seals if recirculation is found. Replace sensor if bias is confirmed after CAC and seal integrity are proven.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Clean external CAC fins if debris packing is found.
- 2Replace CAC if internal restriction or damage is confirmed.
- 3Repair or replace missing air seals between CAC and intake manifold.
- 4Replace IAT sensor if bias is confirmed after CAC and seal integrity are proven.
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 105 FMI 0 is a CAC efficiency and sensing diagnosis. The AI Diesel Tech FSM enforces the true thermal event versus sensor bias separation to prevent unnecessary component replacement.