Before replacing any automotive sensor, answer a short list of questions. What physical condition does the sensor measure? Does the scan value agree with reality? Are the reference voltage, power supply and ground correct? Does the signal change when the physical condition changes? Do related sensors agree? Is there a mechanical fault that could create the same reading? Is the wiring intact from sensor to module? Does replacement require calibration?
Record the evidence. A good diagnostic note might say: ‘ECT reads -40°C on a warm engine. 5 V reference present. Sensor ground voltage drop 0.01 V. Signal wire remains at reference voltage. Jumper test at connector changes scan value according to service procedure. Circuit to ECU verified.’ That is much stronger than ‘P0118 – replaced coolant sensor.’
After repair, verify the original symptom is gone and the data is plausible under the same conditions that produced the fault. If a readiness monitor or self-test is involved, complete the appropriate drive cycle when practical.
The central rule of this book is simple: a sensor code is the start of a test plan, not the end of one.
Figure: Final Diagnostic Checklist