What is the recommended method to test the trailer lights and reflectors on a doubles/triples combination?

Answer: A — Have a helper or use a light test to ensure all lights/reflectors work on each trailer.

The main idea is to actively verify that every trailer in a doubles/triples combination has its lights and reflectors working. The best way to do this is with a helper outside or a portable light tester so you can observe the actual lights on each trailer as you operate the controls. This means turning on headlights, applying the brakes, and using the left and right turn signals to confirm that all lights—taillights, brake lights, turn signals, and any clearance or marker lights—on each trailer illuminate correctly. Relying only on the vehicle’s onboard diagnostics isn’t enough because many trailer lighting issues—bulbs, wiring, or ground faults—may not be detected by the truck’s system. A quick visual check once per trip can miss bulb failures or faulty wiring, and testing only the last trailer leaves the others unchecked, which could lead to unsafe or noncompliant situations.