Fault Detection Principles

The ECU fault detection system is programmed to accept sensor signal values within a certain range to be normal, and signals outside of that range to be abnormal. The normal signal range used to diagnose most sensor circuits covers the entire operating range of the sensor signal. As long as the signal value falls within this range, the ECU judges it to be normal. As a result, it is possible for the sensor to generate a signal which does not accurately represent the actual operating condition and not be detected as a problem by the ECU.

FAULT DETECTION RANGE

FAULT DETECTION RANGE

Abnormal Detection Range

. Normal Range \ For Diagnostic System

THW

Abnormal Detection Range

1

: \

B 'A

Coolant Temperature (°F)

Normal Range y Q

Coolant Temperature (°F)

Normal Range y Q

The fault detection range graph shows typical THW signal parameters. Point A is normal operating temperature and falls within the fault detection normal range. Point B represents the freezing point of water and also falls in the normal range. If the engine is at normal operating temperature but the THW sensor signals the ECU that the coolant temperature is freezing (point B), the engine will operate excessively rich and may not start when hot. Because point B falls within the normal range, the ECU will not recognize this as a problem. No diagnostic code will be set for this problem.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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