Cummins Fault Code 778

21/10/2025

Overview

CODE

REASON

EFFECT

Fault Code: 778
PID: S64
SPN: 723
FMI: 2/2
LAMP: Amber
SRT:

The ECM has detected a loss of signal from the camshaft position sensor.

Possible reduced engine performance.

Cummins Fault Code 778 — Engine Camshaft Speed/Position Sensor Circuit

Circuit Description

The engine control module (ECM) provides a 5 volt supply to the engine camshaft speed/position sensor on the sensor supply circuit. The ECM also provides a ground on the sensor return circuit. The camshaft speed/position sensor provides a signal to the ECM on the camshaft speed/position sensor signal circuit. This sensor generates a signal to the ECM as the camshaft speed indicator lobe moves past the sensor. The ECM interprets this signal into an engine speed reading and determines engine position. This sensor is used as a backup sensor if the primary engine crankshaft speed/position signal is lost.

Component Location

Not Applicable

Conditions for Running the Diagnostics

This diagnostic runs continuously when the engine is running.

Conditions for Setting the Fault Codes

The Engine Control Module (ECM) detected a loss of the camshaft speed/position sensor while the engine was running.

Action Taken When the Fault Code is Active

The ECM illuminates the amber CHECK ENGINE lamp and/or the malfunction indicator lamp (MIL) immediately when the diagnostic runs and fails.

Exhaust gas recirculation (EGR) valve operation will be disabled.

Engine torque will be reduced if the engine is operated for an extended period of time with this fault active.

Conditions for Clearing the Fault Code

To validate the repair, perform a key cycle, start the engine and let it idle for 1 minute.
The fault code status displayed by the recommended Cummins electronic service tool or equivalent will change to INACTIVE immediately after the diagnostic runs and passes.
The ECM will turn off the amber CHECK ENGINE lamp immediately after the diagnostic runs and passes.
For On-Board Diagnostics (OBD) engines, the ECM will extinguish the Malfunction Indicator Lamp (MIL) after three consecutive trips where the diagnostic runs and passes.
The “Reset All Faults” command in the recommended Cummins electronic service tool or equivalent can be used to clear active and inactive faults, as well as extinguish the MIL for OBD applications.