Headlight and Auxiliary Lighting Diagnosis, Aiming, and Replacement

The short version — Know what each bulb/module failure looks like (halogen vs HID vs LED), understand that AFS and auto-leveling are two separate electronic systems that move the beam for different reasons, and remember that aiming always happens on a level vehicle at the correct ride height using the reference marks on the housing.

What these lights are for

• Headlight assemblies produce the low-beam and high-beam patterns that let the driver see the road and let other drivers see the vehicle coming.
• Auxiliary lights (fog lights, driving lights) are supplemental, not primary — they're added for specific situations like low visibility, tight curves, or off-road use. They don't replace the headlight's job; they fill in the beam pattern for a specific condition.

Two different "the beam moved" systems — don't confuse them

This is the part most likely to trip you up on a written test, because both systems change where the light points, but they react to different inputs and for different reasons.

• AFS (Advanced Front-Lighting System) swivels or rotates the headlight housing, or shifts an internal reflector/lens element, based on steering angle, vehicle speed, and sometimes yaw rate. The purpose is to throw light into the turn so you can see around a curve before you're in it. If the AFS actuator motor fails, the headlight will not swivel, may set a DTC, and the AFS function is disabled — but the headlight still lights up normally, it just stops steering with the wheel.
• Automatic beam-leveling uses ride-height or load sensors (could be front, rear, or both) to tell a leveling motor inside the headlight housing to move the beam up or down. This compensates for vehicle load or body pitch — think trunk full of cargo pushing the rear down and the front up, which would otherwise aim the low beam into oncoming traffic's eyes. This leveling function is separate from, and in addition to, manual/static aim adjustment — one is a dynamic in-service system, the other is the fixed baseline aim you set with the vehicle empty and level.
• If the level sensor or leveling motor fails, expect the aim to be stuck too high or too low, or a level warning lamp to come on.

How you actually aim a headlight

• Aiming must be done with the vehicle on a level surface, with correct tire pressure and correct ride height — if the car is sitting wrong, every measurement you take afterward is wrong too, no matter how careful the rest of the procedure is.
• Use either a mechanical aiming screen/wall pattern or an optical aiming machine.
• Line everything up with the horizontal and vertical reference marks, or the bubble level, that are built into or specified for that housing — these marks exist because every housing shape is different, and the marks are the manufacturer's way of telling you "this is level for this specific unit."
• Remember that static aiming and automatic leveling are not interchangeable steps — you still perform the manual aim procedure even on a vehicle equipped with auto-leveling, since the leveling system only compensates for load changes on top of that baseline.

Reading the failure symptoms correctly

Different light sources fail in different, testable ways. Matching the symptom to the source is the core diagnostic skill here.

• Halogen bulb: filament burns out — bulb goes dark, and you may physically see a broken filament if you inspect the bulb.
• HID (high-intensity discharge): failure shows up as flickering, a color shift toward blue/purple, delayed start-up, or total no-light. This can be an arc tube failure in the bulb itself or a ballast failure — the symptoms overlap, so you can't always tell which part failed just by looking at the light from outside.
• LED: failure is a driver failure — you'll see partial segments out (some LEDs dead, others fine) or the entire module dark if the driver itself quits.
• Moisture in the lens: condensation/fogging inside the lens indicates the housing seal has failed. This isn't a bulb or electrical problem — it's a sealing problem that will eventually cause electrical problems if left alone.
• AFS actuator failure: headlight won't swivel, may set a DTC, and AFS is disabled — but basic light output is unaffected.
• Level sensor/motor failure: aim is fixed too high or too low, or the level warning lamp is lit.

Easy to mix up

• HID color shift vs. normal HID color — a blue/purple shift from a formerly white HID beam is a failure symptom (arc or ballast issue), not just "HID looks different from halogen." Don't assume all bluish light is normal.
• AFS vs. auto-leveling — AFS moves the beam side to side into a turn based on steering angle/speed/yaw; auto-leveling moves the beam up and down based on load/ride height. Different sensors, different motors, different purpose. A DTC for one doesn't mean the other is affected.
• LED "partial segments out" vs. HID "total no-light" — an LED module going partially dark points you toward the driver, not the whole module; don't replace the entire assembly before confirming whether it's a driver-only fault.
• Lens fogging vs. bulb failure — fogging is a seal problem, not a symptom you fix by swapping the bulb.

Check yourself

Question: A customer complains that the low beams point noticeably higher when the trunk is loaded with cargo, but they point correctly when the trunk is empty. What system is most likely at fault?

This points to the automatic beam-leveling system — likely a bad ride-height/load sensor or a leveling motor problem. The system is supposed to compensate for load-induced pitch by adjusting beam angle; if it's not correcting it, the sensor or motor isn't doing its job. This is separate from manual static aim, which would show the same problem whether the trunk was loaded or not.

Question: Technician A says a headlight that won't swivel when the driver turns the steering wheel, with a DTC set, indicates an AFS actuator failure. Technician B says the same symptom indicates a beam-leveling motor failure. Who is right?

Technician A is correct. AFS swivels the housing based on steering angle (plus speed and sometimes yaw rate) to light up curves; failure to swivel with a DTC set matches an AFS actuator fault. The leveling motor only handles vertical aim changes for load/pitch compensation, not steering-responsive swivel — so Technician B has the wrong system.

Question: Why must tire pressure and ride height be correct before performing a headlight aim procedure?

Aiming is done relative to a level vehicle. If tire pressure is low or ride height is off, the vehicle's actual level position is different from what the aiming reference marks assume, so any aim you set will be inaccurate once the vehicle is corrected back to normal pressure/height. Getting the baseline right first is what makes the rest of the aim procedure meaningful.