Fan Request, Command, and Airflow Verification
Fan diagnosis is request, electrical command, mechanical response, then airflow path. Thermostats and water pumps replaced while the fan never received a command or never engaged mechanically is the most documented pattern in cooling-system shotgun work.
Fault Overview
This process page sequences overheat and fan complaints into four ordered gates: request, electrical command, mechanical response, and airflow results. No request due to sensor faults, open or short fan circuit, failed clutch, shroud or airflow path issues, and belt issues are the primary causes of fan-related overheat complaints. Each gate must be proven before proceeding to the next.
Engine & Platform Coverage
Symptoms
- •Overheat complaint with no fan response
- •Fan commanded on with no physical engagement
- •ECT rising with fan circuit electrically intact
- •Thermostat or water pump replaced without resolving overheat
- •Shroud missing or damaged reducing airflow
Possible Causes
- 1.No fan request due to ECT or AAT sensor faults
- 2.Open or short fan circuit preventing command from reaching the fan
- 3.Failed fan clutch or drive assembly
- 4.Missing or damaged shroud reducing airflow through the cooling package
- 5.Belt issues on engine-driven fan configurations
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Gate 1 — Validate ECT and AAT: Verify ECT and AAT sensor accuracy before evaluating fan request. A false low ECT reading will prevent the ECM from requesting the fan even when the engine is genuinely hot. Compare ECT to an IR thermometer reading at the thermostat housing.
- 2Gate 2 — Confirm Fan Request: With valid ECT and AAT, verify the ECM is requesting the fan. Check the fan request PID under conditions that should trigger engagement — high ECT, high AAT, or A/C load. If the ECM is not requesting the fan with valid inputs, diagnose the ECM fan control logic.
- 3Gate 3 — Prove Control Voltage and Current: With the fan commanded on, measure control voltage and current at the fan drive connector. Voltage absent confirms an open in the control path. Current absent with voltage present confirms an open load in the fan drive or a disconnected connector.
- 4Gate 4 — Verify Mechanical Engagement: With electrical confirmed, observe physical fan speed or engagement. A fan drive that receives correct voltage and current but does not engage mechanically is a failed fan drive.
- 5Gate 5 — Inspect Shroud and Air Side: Verify the fan shroud is intact and properly sealed to the radiator. A missing or damaged shroud allows air to recirculate around the fan rather than through the cooling package, reducing cooling effectiveness even with a functioning fan.
- 6Gate 6 — Deeper Cooling Diagnostics Only After Fan Is Proven: Proceed to thermostat, water pump, radiator, and cooling system flow diagnostics only after the fan is proven to request, command, engage, and move air correctly. Cooling system parts replaced before this gate is proven are the most common source of overheat comebacks.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Repair ECT or AAT sensor fault preventing fan request.
- 2Repair open or short fan circuit preventing command.
- 3Replace fan clutch or drive assembly after mechanical non-response is confirmed.
- 4Repair or replace missing or damaged fan shroud.
- 5Service belt and drive components if slippage is contributing.
- 6Proceed to thermostat, water pump, and cooling system diagnostics only after fan is 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
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
Related Resources
Related Fault Codes
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Summary
Fan diagnosis is request, electrical command, mechanical response, then airflow path. Prove each gate before proceeding to the next. Cooling system parts replaced before the fan is proven are the most common source of overheat comebacks.