Possible causes

– Timing Reference Optical Sensor harness is open or shorted
– Timing Reference Optical Sensor circuit poor electrical connection
– Faulty Timing Reference Optical Sensor
– Damaged Powertrain Control Module (PCM)

When is the code detected?

Some high resolution pulses are missing from the optical sensor to the PCM

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0377 Description

The optical sensor provides a high resolution signal to the Powertrain Control Module (PCM) by counting pulses on the sensor disk located in the injection pump. The high resolution is one of the most important inputs by the PCM for fuel control and timing. This test monitors the number of high resolution pulses which have been missed (not detected). It’s based on a comparison between the number of pulses that were detected since the last pump cam pulse and the number of the pulses that should have occurred. There are approximately 64 high resolution pulses for every cam pulse.