BOLT 8T5030 - Caterpillar

Each Electronic Control Module (ECM) has the ability to detect problems with the electronic system and with engine operation. When an ECM detects a problem, the ECM generates a code. An alarm may also be generated. There are two types of codes:
Diagnostic
EventDiagnostic Code - When a problem with the electronic system or a component is detected, the Primary ECM or the Secondary ECM generates a diagnostic code. The code indicates the specific problem with the circuitry.Diagnostic codes can have two different states:
Active
LoggedActive Code - An active diagnostic code indicates that an active problem has been detected. Active codes require immediate attention. Always service active codes prior to servicing logged codes.Logged Code - Every generated code is stored in the memory of the ECM that detected the problem. The code is logged. Each ECM can store a maximum of ten different codes at one time. Logged codes remain in the memory of the ECM even if the power is removed from the ECM. Logged codes may be useful to help troubleshoot intermittent problems. Logged codes can also be used to review the performance of the engine and the electronic system. Logged codes may not indicate that a repair is needed. The problem may have been temporary. The problem may have been resolved since the logging of the code. For example, if the system is powered and a sensor is disconnected, an ECM will detect the problem. The ECM will generate an active diagnostic code for the problem. The ECM will also log the code. When the sensor is connected, the active code will disappear. The logged code will remain in the memory of the ECM until the code is cleared.A code is cleared from memory when one of the following conditions occur:
The service technician manually clears the code.
The code does not recur for 1000 hours.
A new code is logged and there are already ten codes in memory. In this case, the oldest code is cleared.Event Code - An event code is generated when an ECM detects 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.