VENT AS 3408819 - Caterpillar

Possible causes for an FMI 3 diagnostic code are:
The signal circuit is shorted to another voltage circuit.
A sensor circuit is open or has a poor connection.
The sensor has failed.
The ECM has failed. A failure of the ECM is unlikely.Possible causes for an FMI 4 diagnostic code are:
The sensor has failed.
The signal circuit in the machine harness is shorted to ground.
The ECM has failed. A failure of the ECM is unlikely.
Illustration 1 g03650642
Schematic of the Analog Active SensorsNote: The diagram above is a simplified schematic of the Machine ECM and the Analog Active connections. The schematic is electrically correct. However, not all of the possible harness connectors are shown. Refer to the latest revision of Electrical Schematic, UENR4017 for the complete schematic.Diagnostic Trouble Code Procedure
Note: Prior to beginning this procedure, inspect the harness connectors that are involved in this circuit. Poor connections can often be the cause of a problem in an electrical circuit. Verify that all connections in the circuit are clean, secure, and in good condition. If a problem with a connection is found, correct the problem and verify that the diagnostic code is active before performing a troubleshooting procedure. For sensors powered via the fuse panel, check the condition of the appropriate fuse before beginning the troubleshooting procedure.
Table 2
Troubleshooting Test Steps    Values    Results    
1. Identify Active DTC Code Associated With Sensor Circuit     Code present     FMI 3 diagnostic code, proceed