Limitations of the Self Diagnostic System

The self diagnostic system provides an excellent routine to direct the technician to the heart of an electronic control system problem. There are however, several limitations which must be kept in mind when troubleshooting.

• The ECU must see a signal in an abnormal range for more than a given amount of time before it will judge that signal to be faulty. Therefore, many intermittent problems cannot be detected by the ECU.

When the ECU stores a diagnostic code, the code indicates a problem somewhere in the sensor circuit, not necessarily in the sensor itself Further testing is always required to properly diagnose the circuit.

Not all circuits are monitored by the ECU. Just because the ECU generates a normal code does not mean that there are no problems within the electronic control system.

Occasionally, diagnostic codes can be set during routine service procedures or by problems outside the electronic control system. Always clear codes and confirm that they reset prior to circuit troubleshooting.

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