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The truck with this event code has lost communication from Access 3 to Access 1. Two things ck for loose power cable connections on Access 3 and then ck CAN communication path with key OFF, ck resistance across CA201 pins 12 & 13k, which are located on the distribution panel you should see 60 ohms and if you see 120 ohms then there is interruption at CA401 pins 1 & 6 on Access 3 or CA215 pins 18 & 19 on back of Access 1. If everything up to this point is good. Disconnect CA215 from Access 1. Then measure resistance across pins 18 & 19 to see that 120 ohms is there. You may see less 118 ohms should be min. If the resistance isn't in this range Access 1 needs to be replaced.
Disconnect CA401 on Access 3 put a jumper between CA401-2 and CA401-4 on Access 3 measure resistance across CA401 pins 6 & 1. You should come up with 118/120 ohms. If you don't see the resistance in this range Access 3 needs to be replaced. Also ck the CAN low line for continuity from CA401 pin 6 harness connector to CA215 pin 18 harness connection, and high CAN line continuity CA401 pin 1 to CA215 pin 19.