A construction site technician is diagnosing a fault on a dump truck. The truck's dashboard display occasionally shows 'CAN Bus Communication Error' for the engine control unit (ECU). When checking the CAN bus, there are no visible signs of damage to the wiring. The technician connects an oscilloscope and observes intermittent signal reflections on the CAN High and CAN Low lines, particularly when the truck hits bumps. What is the most probable cause?

Question: A construction site technician is diagnosing a fault on a dump truck. The truck's dashboard display occasionally shows 'CAN Bus Communication Error' for the engine control unit (ECU). When checking the CAN bus, there are no visible signs of damage to the wiring. The technician connects an oscilloscope and observes intermittent signal reflections on the CAN High and CAN Low lines, particularly when the truck hits bumps. What is the most probable cause?

Answer options:

• A faulty ECU module. ✅ Intermittent open circuit in a CAN bus termination resistor.
• A short circuit between CAN High and CAN Low lines.
• Incorrect CAN bus data rates between connected modules.

Correct answer: Intermittent open circuit in a CAN bus termination resistor.

Explanation: Signal reflections, especially when intermittent and triggered by physical impact (like hitting bumps), are a strong indicator of a compromised termination resistor or connection at the end of the CAN bus. Termination resistors absorb signals to prevent reflections. If one is intermittently open, signals will reflect, causing communication errors. A faulty ECU would likely cause a continuous error or specific functional issues. A short between CAN High and CAN Low would result in a constant communication failure with flatlined signals. Incorrect data rates would cause continuous, not intermittent, communication errors.