SPN 4360 FMI 1 — SCR Intermediate Temperature Below Normal
Low SCR intermediate temperature is sensing or thermal-state diagnosis first. Unplugged sensors after aftertreatment service and open circuits from brittle harnesses are the most documented patterns. The FSM enforces sensor correlation before SCR hardware decisions.
Fault Overview
SPN 4360 FMI 1 indicates SCR intermediate or mid-bed temperature below expected where the sensor is used. Failed temperature sensor, open circuit, exhaust leak near the sensor boss, low-load duty cycle, and upstream thermal management failure are the primary causes. Unplugged sensors after aftertreatment service and open circuits from brittle harnesses are documented patterns. Replacing the SCR catalyst for a temperature sensor fault is the most documented misdiagnosis pattern.
Engine & Platform Coverage
Symptoms
- •SCR mid temp low
- •SCR temperature sensor low
- •Efficiency low from cold SCR
- •SCR not reaching operating temperature
- •SCR intermediate temperature below expected
Possible Causes
- 1.Failed temperature sensor — failed EGT sensor producing stuck-low reading
- 2.Open circuit — open circuit in EGT sensor harness producing stuck-low reading
- 3.Exhaust leak near boss — exhaust leak near sensor boss producing false low reading
- 4.Low-load duty cycle — low-load duty cycle preventing SCR from reaching operating temperature
- 5.Upstream thermal management failure — upstream thermal management failure preventing SCR from reaching temperature
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Multi-Temperature Correlation: Correlate the SCR intermediate temperature with SCR inlet and outlet temperatures. One stuck-low sensor is suspect. A single sensor reading significantly lower than adjacent sensors confirms a sensor or circuit fault. All sensors reading low confirms a thermal state or upstream issue.
- 2Step 2 — Circuit Continuity Check: Check circuit continuity on the suspect sensor. Unplugged sensors after aftertreatment service are a documented pattern — always inspect the EGT connector and circuit after any recent aftertreatment service. Verify continuity through the sensor circuit. An open circuit confirms a harness or connector fault.
- 3Step 3 — Inspect for Exhaust Leaks Near Boss: Inspect for exhaust leaks near the sensor boss. An exhaust leak near the sensor boss produces a false low reading by allowing ambient air to cool the sensor. Inspect the sensor boss and surrounding exhaust connections for leaks.
- 4Step 4 — Evaluate Duty Cycle and Thermal Strategy: Evaluate the duty cycle and upstream thermal management strategy. A low-load duty cycle preventing the SCR from reaching operating temperature produces a legitimate low temperature reading that is not a sensor fault. Evaluate whether the duty cycle is appropriate for SCR operation.
- 5Step 5 — Sensor and Harness Repair: Repair the sensor or harness if a fault is confirmed. Replace the sensor if failure is confirmed. Repair the harness if an open circuit is confirmed. Repair the exhaust leak if a leak is confirmed.
- 6Step 6 — Verify After Repair: Verify SCR intermediate temperature readings are correct after repair. Confirm SPN 4360 FMI 1 does not return. Monitor for return of low temperature events over extended operation.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair connector or harness if open circuit is confirmed.
- 2Replace EGT sensor if failure is confirmed.
- 3Repair exhaust leak if leak near sensor boss is confirmed.
- 4Address upstream thermal management if thermal strategy failure is confirmed.
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
Low SCR intermediate temperature is sensing or thermal-state diagnosis first. The AI Diesel Tech FSM enforces sensor correlation before SCR hardware decisions.