Check Open Circuit

For the open circuit in the wire harness in Figure, perform "(a) Continuity Check" or "(b) Voltage Check" to locate the section.

Fig 1: Checking Continuity

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Check the continuity.
1. Disconnect connectors "A" and "C" and measure the resistance between them.
  In the case of Fig 2:
  
  Between terminal 1 of connector "A" and terminal 1 of connector "C" --> No continuity (open) Between terminal 2 of connector "A" and terminal 2 of connector "C" -> Continuity
  
  Therefore, it is found out that there is an open circuit between terminal 1 of connector "A" and terminal 1 of connector "C".
  
  Fig 2: Disconnecting Connectors "A" And "C" And Measuring Resistance
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2. Disconnect connector "B" and measure the resistance between the connectors.
  In the case of Fig 3:
  
  Between terminal 1 of connector "A" and terminal 1 of connector "B1" --> Continuity
  
  Between terminal 1 of connector "B2" and terminal 1 of connector "C" --> No continuity (open)
  
  Therefore, it is found out that there is an open circuit between terminal 1 of connector "B2" and terminal 1 of connector "C".
  
  Fig 3: Disconnecting Connector "B" And Measuring Resistance Between Connectors
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Check the voltage.
In a circuit in which voltage is applied (to the ECU connector terminal), an open circuit can be checked for by conducting a voltage check.
1. As shown in Fig 4, with each connector still connected, measure the voltage between body ground and terminal 1 of connector "A" at the ECU 5V output terminal, terminal 1 of connector "B", and terminal 1 of connector "C", in that order.
Fig 4: Measuring Voltage Between Body Ground And Terminal 1 Of Connector "A" At ECU 5V Output Terminal
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If the results are:

5V: Between Terminal 1 of connector "A" and Body Ground

5V: Between Terminal 1 of connector "B" and Body Ground

0V: Between Terminal 1 of connector "C" and Body Ground

Then it is found out that there is an open circuit in the wire harness between terminal 1 of "B" and terminal 1 of "C".