The Class II serial data link is configured in a ring used to communicate information between control modules. The controllers on the Class II link use a variable pulse width modulated square wave of 0-7 volts for communication. Zero volts equals a low state or logic 0 and 7 volts equals a high state or logic 1. The body control module (BCM), powertrain control module (PCM) - L61 only, and sensing and diagnostic module (SDM) monitor the Class II link for low and high voltage faults.

DTC U1300 will set if the BCM, PCM, or SDM detected the Class II circuit in the low state for too long. The Class II circuit is shorted to ground when:

DTC U1300 runs continuously with the ignition ON.

DTC U1300 is a type C DTC.

If the scan tool can communicate with either the BCM, PCM, or SDM, then the Class II link is not currently shorted to ground.

Important: The SDM will set U1300 for any fault related to the Class II link circuit. Therefore, if the link is shorted to voltage, the SDM will set U1300 while the BCM sets U1301.

Inspect the wiring harness for signs of chafing or broken insulation.

To locate an intermittent problem, use the scan tool to monitor any of the controllers on the Class II link located under the DIAGNOSTIC CIRCUIT CHECK - CLASS II MESSAGE MONITOR with the ignition ON and the engine OFF. INACTIVE will be displayed on all controllers if the link is shorted to ground. NOTE: The controller name will not be displayed until the scan tool initially received the correct controller ID over the link upon ignition ON.