Posted on October 14, 2020 C13, C15, and C18 Engines – Diagnostic Codes The engines Electronic Control Module (ECM) can monitor the circuitry between the ECM and the engines components. The ECM can also monitor the engines operating conditions. If the ECM detects a problem, a code is generated. There are two categories of codes: Diagnostic code Event code Diagnostic Code – A diagnostic code indicates an electrical problem such as a short circuit or an open circuit in the engines wiring or in an electrical component. Event Code – An event code is generated by the detection of an abnormal engine operating condition. For example, an event code will be generated if the oil pressure is too low. In this case, the event code indicates the symptom of a problem. Generally, event codes indicate abnormal operating conditions or mechanical problems rather than electrical problems. Codes can have two different states: Active Logged Active Codes An active code indicates that a problem is present. Service the active code first. For the appropriate troubleshooting procedure for a particular code, refer to the following troubleshooting procedure: Troubleshooting, “Diagnostic Trouble Codes” Troubleshooting, “Event Codes” Logged Codes The codes are logged and stored in the ECM memory. The problem may have been repaired and/or the problem may no longer exist. If the system is powered, an active diagnostic code may be generated whenever a component is disconnected. If the component is reconnected, the code is no longer active but the code may become logged. Logged codes may not indicate that a repair is needed. The problem may have been temporary. Logged codes may be useful to help troubleshoot intermittent problems. Logged codes can also be used to review the performance of the engine and of the electronic system. An additional status screen is available for the Enhanced Troubleshooting Indicator ETI. The screen is accessed through Caterpillar Electronic Technician (ET).