3126B Generator Set Engine Caterpillar

Diagnostic Codes

Diagnostic codes alert the operator that a problem in the electronic system has been detected. Diagnostic codes also indicate the nature of the problem to the service technician. The Caterpillar Electronic Technician (ET) is a software program that is designed to run on a personal computer. Diagnostic codes may be viewed on a personal computer that has Cat ET software.

Component Identifier (CID) - The CID is a number with three or four digits. The CID indicates the component that generated the code. For example, the CID number 0001 identifies the fuel injector for the number one cylinder.

Failure Mode Identifier (FMI) - The FMI is a two digit code that indicates the type of failure.

Note: Do not confuse diagnostic codes with event codes. For information on event codes, see Troubleshooting, "Event Codes".

Illustration 1	g01100964
Output voltage from a typical analog temperature sensor

Illustration 1 indicates the signal range for a typical analog sensor. Diagnostic codes will be generated if the sensor's output signal is below 0.2 VDC or above 4.8 VDC.

Active Diagnostic Codes

An active diagnostic code represents a problem with the electronic control system. Correct the problem as soon as possible.

If the condition that generated the code is momentary, the message disappears from the list of active diagnostic codes. The diagnostic code becomes logged.

Logged Diagnostic Codes

When the Electronic Control Module (ECM) generates a diagnostic code, the ECM logs the code in permanent memory. The ECM will record the following information when a code is generated:

• The hour of the first occurrence of the code

• The hour of the last occurrence of the code

• The number of occurrences of the code

This information is a valuable indicator for troubleshooting intermittent problems.

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 100 hours.

• A new code is logged and there are already ten codes in memory. In this case, the oldest code is cleared.

Some diagnostic codes may be easily triggered. Some diagnostic codes may log occurrences that did not result in complaints. The most likely cause of an intermittent problem is a faulty connection or damaged wiring. The next likely cause is a component failure. The least likely cause is the failure of the engine's ECM. Diagnostic codes that are logged repeatedly indicate a problem that needs special investigation.

Note: Always clear logged diagnostic codes after investigating and correcting the problem which generated the code.

Diagnostic Codes Table