Verify the electronic control module (ECM) calibration is correct. Check the calibration revision history found on QuickServe™ Online for applicable fixes to the calibration stored in the ECM. If necessary, calibrate the ECM.

During engine operation, the ECM monitors the secondary speed sensor (camshaft speed sensor) signal. Fault Code 778 will become active and light the yellow lamp if the signal from the secondary speed sensor is not present or degraded for more than 3 continuous seconds. If, over a period of time, the ECM detects several incidents of a loss of the secondary speed sensor signal, but the signal loss is short in duration, this fault code will become active. Since the secondary engine speed signal is not used for normal engine operation, a vehicle operator should not notice any symptoms related to this problem.

This fault code becomes active whenever the ECM detects a persistent loss of the secondary engine speed sensor signal over a short period of time.

This fault becomes inactive at each key-ON or if the ECM does not detect a loss of the secondary engine speed sensor signal for at least 20 minutes.

Possible causes:

1)Intermittent loss of primary speed sensor signal; the most likely cause is an intermittent open or short circuit that can be experienced with speed sensor connector problems
2)Sensor air gap that is marginally too small or too large, or target wheel exhibits excessive runout.

Actions to take if this fault is intermittent:

1)Look for intermittent engine harness connections in the secondary engine speed sensor circuit
2)Look for loose connections at the ECM connector where the pins can not be locked into the socket
3)Look for places where the engine harness has worn into a nearby component, causing an intermittent short circuit
4)Look for pin wear at the speed sensor
5)Inspect the target wheel for damage.

Possible repairs:

1)Repair or replace the engine harness.
2)Replace or shim the sensor if the air gap is out of specification. If the target runout is excessive, replace the target.
3)Replace the engine camshaft speed/position sensor.