Data symbols (1's and 0's) are transmitted sequentially. The data to be transmitted over the bus is represented by the voltage difference between the CAN-High signal voltage and the CAN-Low signal voltage. When a logic '0' is to be transmitted, the High signal circuit is driven higher to about 3.5 V and the Low circuit is driven lower to about 1.5 V. The differential voltage becomes approximately 2.0 (+/- 0.5) V. When the PCM detects a determined amount of communication errors or no communication, a DTC U0001 is set.