A technician is diagnosing a complaint that the vehicle's rear license plate lights are inoperative. The taillights, parking lights, and dash illumination lights all work correctly. A voltage reading at one of the license plate light bulb sockets shows 0V when the parking lights are on. The fuse for the license plate lights is confirmed to be good. What is the MOST likely cause?
- ABoth license plate light bulbs are burnt out.
- BAn open circuit in the power wire supplying power from the parking light circuit to the license plate lights.
- CAn open circuit in the common ground wire for the license plate lights.
- DA faulty headlight switch or parking light relay.
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit in the power wire supplying power from the parking light circuit to the license plate lights.
Since the taillights and parking lights (which typically share the same circuit as license plate lights) are working, the main headlight switch/relay and fuse are good. The 0V at the license plate bulb socket indicates a loss of power to those specific lights. This points to an open circuit in the wiring branch that specifically feeds the license plate lights after it splits from the main parking/taillight circuit.
Why the other options are wrong
- A. If both bulbs were burnt out, there would still be voltage present at the bulb sockets, but the lights wouldn't illuminate. The question states 0V.
- C. If the ground wire was open, a voltmeter connected from the power wire at the bulb socket to chassis ground would show voltage. Since 0V is measured, the power feed is missing.
- D. The headlight switch or parking light relay is confirmed to be working because the taillights and parking lights are operational. If it were faulty, it would affect all lights on that circuit, not just the license plate lights.
Branch Circuit Open
A branch circuit open occurs when a specific segment of wiring that splits off from a main circuit has a break, causing only the components on that branch to fail while the main circuit and other branches remain operational.
- Common in lighting systems where multiple components share a main power feed.
- Diagnosis involves tracing power from the main circuit split point to the affected branch.
- Often caused by chafed wires, corroded connectors, or physical damage.
Memory trick: Plate lights gone? Power's branch is wrong!