Lighting is 6 of the 50 scored A6 questions. The 2026 task list goes past bulbs and switches: HID, LED and adaptive front lighting, aiming and leveling, module-controlled lamps, and trailer wiring with brake control.
Trailer wiring: harness, relays, connectors and modules, including brake control.
Symptoms and where they point
Dim lamp: resistance on the power or ground side. A voltage drop test with the lamp on finds it.
Brighter than normal: too much voltage, which sends you to the charging system, or a lamp of the wrong rating.
One lamp lights another: a bad ground lets current return through the other filament of a dual-filament bulb, so the brake and tail lamps or the turn and park lamps glow together.
Fast flash on one side: with a load-sensing flasher, a lamp out on that side. After a swap to LED bulbs the load is too low, which needs an LED-rated flasher or a load resistor. A module-controlled system may log a code or show a lamp-out message instead.
Lamp stays on: a switch or relay stuck closed, a short to power, or a module commanding it on; the scan tool shows which.
Trailer lamps: check the vehicle side at the connector first, then the trailer's own ground.
Trap: testing an LED or HID lamp like a bulb
An ohmmeter across an LED assembly or an HID bulb tells you little, and the ballast output that starts an HID lamp is high voltage you should not probe. Check supply and ground at the connector, then follow the manufacturer's procedure for the lamp or ballast.
13 lighting questions
0 of 13 answered · 0 correct
Question 1 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
A vehicle's brake light stays on dimly whenever the headlights are turned on, even with the brake pedal released. The brake light and headlight circuits share a common ground splice. What is the most likely cause?
Explanation
Incorrect: A switch stuck closed lights the brake lamps fully, headlights on or off.
Correct: With the shared ground open, headlight-circuit current finds its way to ground through the brake bulb filament and lights it dimly.
Incorrect: A short to power lights the brake lamps at full brightness all the time.
Incorrect: A headlight short to ground would blow its fuse, not light the brake lamp.
Incorrect: Resistive switch contacts dim the brake lamps when braking; they cannot switch them on with the headlights.
Question 2 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
All brake lights on a vehicle remain on constantly, even with the ignition off. The most likely cause is:
Explanation
Incorrect: Most stop lamp circuits use no relay, and the pedal switch is the usual failure.
Incorrect: A taillight short affects the tail lamps or blows a fuse; it does not light every brake lamp.
Incorrect: With the key off most modules are asleep, which makes the pedal switch the likely cause.
Correct: Stop lamps are usually fed from an always-hot fuse through the pedal switch, so a switch stuck closed keeps them on with the key off.
Question 3 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
A customer complains that the interior courtesy lights stay on after all doors are closed. Which of the following is the most likely cause?
Explanation
Incorrect: An open ground leaves the courtesy lamps dark, not on.
Incorrect: A blown dome fuse turns the courtesy lights off, the opposite of the complaint.
Incorrect: A burned-out bulb leaves that lamp dark, not lit.
Correct: A door ajar switch stuck closed tells the circuit a door is open, so the courtesy lights stay on.
Question 4 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
When aiming headlights using a headlight aiming screen, what should be checked before beginning the procedure?
Explanation
Incorrect: Lens clarity affects light output, but aim also depends on ride height.
Incorrect: Battery voltage affects brightness, not where the beam points.
Incorrect: Bulb specifications matter for replacement, not for setting aim.
Correct: Tire pressure and load change ride height and pitch, which moves the beams, so both are set before aiming.
Question 5 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
What causes rapid turn signal blinking?
Explanation
Incorrect: A worn cancel cam keeps the lever from returning; it does not change the flash rate.
Correct: A burned-out bulb lowers the load on the flasher circuit, so that side blinks fast.
Incorrect: Low voltage slows a thermal flasher, the opposite of rapid blinking.
Incorrect: A failed flasher usually stops flashing or stays lit; it does not explain fast blinking.
Question 6 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
How are the door jamb switches that control an interior dome light wired relative to one another?
Explanation
Correct: Each door switch is in parallel with the others, so opening any one door completes the lamp circuit.
Incorrect: In series every door would have to be open at once to light the lamp.
Incorrect: The switches all control the same lamp, so they are not independent circuits.
Incorrect: The door switches are simple parallel branches, not a complex network.
Question 7 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
A customer complains that their low beam headlights work but the high beams do not. Technician A says the multifunction switch could be faulty. Technician B says a blown high beam fuse could be the cause. Who is correct?
Explanation
Incorrect: Technician A is right about the multifunction switch, but Technician B's high-beam fuse is also a valid cause.
Incorrect: Technician B is right about the fuse, but a faulty multifunction switch can also kill the high beams alone.
Correct: Both are right: the high beams have their own switch contacts and often their own fuse or relay, which the low beams do not share.
Incorrect: The switch contacts and a high-beam fuse can each cause this symptom.
Question 8 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
A vehicle with HID (high-intensity discharge) headlights has one headlight that flickers and sometimes does not turn on. Swapping the bulbs side to side does not move the problem, and the connectors are tight. The most likely cause is:
Explanation
Incorrect: A blown fuse would keep the lamp off all the time instead of flickering.
Incorrect: The switch feeds both headlights, so it would not affect one side.
Incorrect: The connectors were checked and are tight, which rules out a loose connection.
Correct: The bulb swap and tight connectors leave that side's ballast, the usual cause of HID flicker.
Question 9 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
What safety precaution must be taken when working with HID lighting systems?
Explanation
Correct: HID ballasts make very high starting voltage, so the lamps are switched off and the ballast allowed to discharge before handling.
Incorrect: Running the lamps energizes the ballast and heats the bulb, the opposite of a safe state.
Incorrect: The chassis is already ground; grounding it removes no ballast voltage.
Incorrect: Sunglasses offer no protection from the shock hazard of the ballast.
Question 10 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
How is current draw measured in a taillight circuit?
Explanation
Incorrect: A voltmeter at the bulb reads the voltage reaching it, not the current through it.
Incorrect: An ohmmeter measures resistance on a dead circuit, not current.
Incorrect: A test light shows that voltage is present but gives no current value.
Correct: Current is read by making the circuit flow through the ammeter, which needs a series connection.
Question 11 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
How should reverse light circuit voltage be tested?
Explanation
Correct: Testing at the socket with the switch closed (key on, in reverse) checks the whole feed through the switch and wiring.
Incorrect: The switch output does not prove the wiring between the switch and the lamp.
Incorrect: Voltage at the fuse box does not prove that power reaches the lamp.
Incorrect: The engine need not run, and the circuit stays off unless the transmission is in reverse.
Question 12 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
After replacing a headlight bulb on a vehicle with automatic headlight leveling, the headlight beam points too low. The MOST appropriate action is to:
Explanation
Correct: Automatic leveling has to be initialized after service, which resets its zero point, usually with a scan tool.
Incorrect: A general BCM reset does not run the leveling calibration.
Incorrect: Mechanical aim is set only after leveling is calibrated; turning the screws first hides the fault.
Incorrect: A new bulb does not change beam angle; the leveling system sets it.
Question 13 of 13 · Lighting Systems Diagnosis and Repair · 12%Not answered
What most often makes an LED lamp lose brightness permanently over its service life?
Explanation
Incorrect: Low supply voltage dims the lamp only while it lasts; output returns once voltage is corrected.
Incorrect: A dirty lens cuts light until it is cleaned, but the LED itself is not degraded.
Incorrect: Extra resistance dims the lamp only while it is present; repairing the circuit restores full output.
Correct: LED output fades for good when the junction runs hot, so a poor heat sink ages the lamp early.
With a load-sensing flasher, a fast flash on one side means less load than expected: a lamp out, a bad socket or ground, or LED bulbs fitted where the flasher expects incandescent ones.
Is headlight aiming on the A6 test?
Yes. The lighting task list includes aiming and leveling headlamp and auxiliary lamp assemblies, HID, LED and adaptive systems included.