Service function guideThermostat and Fan Diagnosis

This guide delivers a complete diagnostic procedure for any engine that runs too hot or never reaches operating temperature, teaching you to separate thermostat faults from fan circuit failures using pressure, temperature and electrical tests. Written for working mechanics and serious DIY owners, it walks through system operation, preparation, step-by-step testing, result confirmation, vehicle-specific variations, common mistakes and aftercare. By the end you will be able to prove the root cause with measured evidence rather than parts-swapping, and you will know exactly which component or circuit to repair. Every test is described with the sounds, readings and visual cues you should expect, so you can document a defensible diagnosis on any make or model.