Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a communication issue on a CAN bus system. Using a digital multimeter, the technician measures the voltage between CAN High and ground, and CAN Low and ground, with the ignition on. What are the typical expected voltage readings for CAN High and CAN Low on a properly functioning high-speed CAN bus at rest (no active communication)?
- ACAN High: 2.5V, CAN Low: 2.5V
- BCAN High: 5.0V, CAN Low: 0V
- CCAN High: 0V, CAN Low: 0V
- DCAN High: 3.5V, CAN Low: 1.5V
Show answer & explanationAnswer & explanation
Correct answer: A. CAN High: 2.5V, CAN Low: 2.5V
On a high-speed CAN bus (ISO 11898-2), when the bus is idle or 'at rest' (recessive state), both CAN High and CAN Low lines are typically biased to approximately 2.5 volts. This common mode voltage helps with noise immunity. During active communication (dominant state), CAN High goes to approximately 3.5V and CAN Low goes to 1.5V.
Why the other options are wrong
- B. 5.0V and 0V represent a single-wire communication system, not a differential CAN bus.
- C. 0V on both lines would indicate an open circuit or no power to the bus.
- D. 3.5V and 1.5V are the typical voltages during the 'dominant' state (active communication), not at rest.
CAN Bus Recessive State
The 'at rest' or idle state of a high-speed CAN bus, where both CAN High and CAN Low lines are biased to approximately 2.5 volts, indicating no module is actively transmitting data.
- Both lines at 2.5V.
- Indicates idle bus.
- Differential voltage is 0V.
Memory trick: CAN lines at rest, 2.5V is the neutral, a calm bus waiting for talk.