SPN 105 FMI 1 — Intake Manifold Temperature Below Normal
SPN 105 FMI 1 means the intake manifold temperature is below the expected minimum. This is almost always a sensing or circuit fault — a cold-soak ambient correlation check is the mandatory first gate before any fuel system or boost system diagnosis.
Fault Overview
FMI 1 on intake manifold temperature indicates the sensor reports below the minimum expected temperature for conditions. Failed sensor, open circuit, connector corrosion, and incorrect sensor installation are the primary causes. A sensor left unplugged after intake service is a documented cause. If the engine heat-soaks and the IAT never rises above ambient, the sensor or circuit is the fault — not the fuel or boost system. Performing fuel system or boost system diagnosis based on a false cold IAT reading is the primary wasted-time pattern on this fault.
Engine & Platform Coverage
Symptoms
- •Intake temp stuck low fault active
- •IAT sensor cold indication
- •Fueling rich smoke from false cold IAT
- •Boost control edge cases from cold IAT bias
Possible Causes
- 1.Failed sensor — thermistor failure producing cold bias
- 2.Open circuit — open signal or ground producing cold bias
- 3.Connector corrosion — high-resistance connection producing cold bias
- 4.Incorrect sensor installation — wrong sensor or improper seating
- 5.Sensor left unplugged after intake service
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 — Cold-Soak Ambient Correlation: Compare IAT reading to ambient temperature at cold soak. IAT should be near ambient at cold soak. If IAT reads significantly below ambient at cold soak, the sensor or circuit is biased cold. If IAT reads near ambient at cold soak but never rises during operation, the sensor is stuck.
- 2Binary Decision Gate — Sensor Connection Verification: Verify the sensor is connected and properly seated. A sensor left unplugged after intake service produces a cold bias that triggers FMI 1. Inspect the sensor connector for proper engagement and pin condition.
- 3Override Manager — Harness Continuity Check: Perform continuity test on the IAT sensor circuit. An open signal or ground produces a cold bias reading. Inspect connector for corrosion that creates high resistance mimicking an open circuit.
- 4Safety Interceptor — Sensor Resistance vs Temperature Chart: Measure sensor resistance and compare to OEM temperature-resistance curve if available. A sensor reading outside the expected resistance for current temperature confirms sensor failure.
- 5Resume Point — Operating Temperature Rise Assessment: If cold-soak reading is near ambient but IAT never rises during operation, the sensor is stuck at a cold value. This confirms sensor failure rather than a circuit fault.
- 6Verification Loop — Post-Repair Temperature Confirmation: After repair, verify IAT rises appropriately during operation and correlates with ambient at cold soak. Confirm no smoke or boost control complaints remain after repair.
- 7Repair Ledger Output — Sensor or Harness Decision: Replace sensor after circuit integrity is confirmed and sensor failure is proven by resistance test or stuck reading. Repair harness if open circuit or corrosion is found.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Reconnect sensor if found unplugged after intake service.
- 2Repair connector corrosion or open circuit.
- 3Replace IAT sensor after circuit proof confirms sensor failure.
- 4Verify correct sensor installation if incorrect sensor is suspected.
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.
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Summary
SPN 105 FMI 1 is almost always a sensing or circuit fault. The AI Diesel Tech FSM enforces the cold-soak plausibility gate to prevent fuel system guesswork from a false cold IAT reading.