Theory Of Operation

The battery voltage supply circuit to the Electronic Control Unit (ECU) is monitored to ensure the proper voltage range is supplied to operate the ECU within its design limits. Vehicle system voltage is also monitored by the ECU through the Controller Area Network (CAN).

If the battery supply circuit voltage is out of range and the vehicle system voltage message received over CAN is normal, the problem is most likely with the ECU battery supply circuit or ground circuit. If the battery supply circuit voltage is out of range and the vehicle system voltage message received over CAN is also out of range, the problem is most likely with the vehicle battery/charging system.

The Ignition voltage supply circuit is used to "wake" the ECU when the vehicle ignition is turned on. This circuit is typically not monitored for voltage supply issues. If the Ignition supply circuit is below a certain voltage, the ECU will not "wake" and therefore cannot set voltage DTCs. Other ECUs in the vehicle will however set lost communication DTCs since the ECU will not be communicating on the CAN.