SWITCH GP-ROCKER 8Y7237 - Caterpillar

Diagnostic trouble codes are generated by the Electronic Control Module (ECM) to alert the operator that a problem has been detected. The problem can be in the electronic system or in the mechanical system. Diagnostic trouble codes are used by the service technician to assist in the identification of the nature of the problem. Caterpillar Electronic Technician (ET) is a software program that can be used by the service technician to view the diagnostic code information. Refer to Troubleshooting, "Electronic Service Tools" for detailed information that is related to the functionality of Cat ET.The CAN data link is used by the ECM to communicate with controllers, display modules, and electronic service tools. The ECM transmits information and the ECM receives information over the CAN data link. The communications protocol and the diagnostic messages over the CAN data link follow the SAE J1939 industrial standards.Note: The SAE J1939 industrial standards refer to the control module for the engine as the Electronic Control Unit (ECU). This manual refers to this control module as the Electronic Control Module (ECM).Communications with electronic service tools aid the technician in diagnostics and troubleshooting for the engine and the engine subsystems. When a problem with the engine is detected by the ECM, the ECM communicates information about the problem over the CAN data link. The problem may be physically detected by one of the input or output circuits by the ECM. A problem can also be communicated via the data link to the ECM. The ECM transmits this information over the data link in a form of a diagnostic trouble code and code description.The diagnostic trouble code has a format of a two-element code. The following terms describe the elements of the code:Suspect Parameter Number (SPN) - The SPN is a code which is assigned to each component. The SPN identifies the origin of the problem.Failure Mode Identifier (FMI) - The FMI is used with the SPN to provide more information that relates to the problem. The FMI may indicate that a problem with an electronic circuit or an electronic component has been detected. The FMI may also indicate that an abnormal operating condition has been detected.The codes are displayed in the form “SPN-FMI”. The ECM also attaches a text description to the message that is transmitted over the data link. This text description is used to describe the SPN-FMI code.The failure mode identifiers can be divided into two categories:
Codes that indicate that a problem with an electronic circuit or an electronic component has been detected
Codes that indicate that an engine event or a system event has been detectedThe failure mode of the fault is determined by evaluating the electronic signal from the suspect circuit. Typically, the first category of code is generated when the signal for the circuit is outside the range of the sensor. The second category of code is an engine event. An engine event indicates that the sensor signal is OK, but the signal is outside the normal operating range of