SPN 110 FMI 0 — Engine Coolant Temperature Above Normal (Most Severe)
SPN 110 FMI 0 means engine coolant temperature has exceeded the most severe calibrated threshold. Overheat protection logic is active or pending. Stop the vehicle if temperature is severe. Verify whether this is a true thermal event or a sensor issue before any major engine work.
Fault Overview
The ECT sensor (SPN 110) monitors coolant temperature for engine protection. FMI 0 (most severe) triggers when temperature exceeds the highest protection threshold. True overheat can cause head gasket failure, liner distortion, and catastrophic engine damage. However, a failed ECT sensor or air pocket after service can mimic overheat without actual thermal damage. Separate true overheat from sensor fault before any teardown.
Engine & Platform Coverage
Symptoms
- •Engine overheat warning on dash
- •Coolant temperature gauge in red zone
- •Derate or engine protection shutdown
- •Fan running continuously or not engaging
- •Steam from coolant system
- •Coolant loss
Possible Causes
- 1.Low coolant level — leak or consumption
- 2.Failed thermostat — stuck closed
- 3.Weak water pump — insufficient flow
- 4.Clogged radiator or CAC package — debris or scale
- 5.Fan clutch failure — not engaging at temperature
- 6.Fan modulator/control fault
- 7.ECT sensor bias — false high reading
- 8.Air pocket after service — false overheat reading
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 vs True Thermal Separation: Compare ECT gauge reading to an infrared temperature measurement at the sender location. A large discrepancy suggests sensor bias rather than true overheat.
- 2Binary Decision Gate — Coolant Level & Pressure Test: Check coolant level and perform a pressure cap test. Low level or failed cap is the most common cause of true overheat.
- 3Override Manager — Combustion Gas Check: If coolant loss is unexplained, test for combustion gases in the coolant (block test or combustion leak test). Positive result indicates head gasket or liner issue.
- 4Safety Interceptor — Fan Command & Operation: Verify fan is being commanded on at temperature and is actually engaging. Fan clutch failure or modulator fault is a common cause of overheat at low speed.
- 5Resume Point — Thermostat Differential Test: Measure temperature differential across the thermostat. A stuck-closed thermostat will show a large temperature difference between inlet and outlet.
- 6Verification Loop — Airflow Package Inspection: Inspect radiator, CAC, and condenser for debris packing, bent fins, or scale buildup. Restricted airflow causes overheat at high ambient or load.
- 7Repair Ledger Output — Major Engine Work Decision: Avoid head gasket or major engine work without positive combustion leak test or physical evidence of failure. Document all findings.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
5 steps locked- 1Repair coolant leaks and restore proper level.
- 2Replace failed thermostat if stuck closed.
- 3Replace fan clutch or repair fan modulator if fan is not engaging.
- 4Clean or replace radiator/CAC if airflow is restricted.
- 5Replace water pump if flow is insufficient.
- 6Replace ECT sensor if bias is confirmed.
- 7Perform head gasket or major engine repair only with positive combustion leak test evidence.
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.
Related Resources
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Summary
Treat severe over-temp as an immediate protection issue. Verify true overheat versus sensor bias before any teardown. The AI Diesel Tech FSM enforces sensor separation and combustion leak testing before major engine work decisions.