Lighting and Signalling Systems
Bulbs, control and beam pattern
Know the families: halogen (tungsten filament, do not touch the glass with bare fingers), LED (low current, often driven by a module or PWM), and HID/xenon (a ballast produces a high-voltage strike, so never work on a live circuit). Many vehicles switch lamps by PWM or by a body control module rather than a mechanical switch, and monitor bulb current so they can flag a failure.
Circuits are usually parallel, so each lamp sees full system voltage and currents add: a 21 W lamp at 12 V draws 1.75 A. Use that arithmetic to check whether a fuse and wire are correctly sized, and to prove an unexpected current draw.
Signalling covers indicators and hazard flashers (flasher unit or BCM-controlled), brake and reverse lamps, side markers, fog lamps, DRLs and trailer lighting. Beam pattern matters: headlamp aim must be set on a proper beam setter with correct tyre pressures and load, and left-hand/right-hand drive masks differ. Common complaints are hyperflash after an LED conversion, a bulb-out warning from a low-current lamp, and one indicator flashing fast because a lamp or earth is faulty.
Traps
Assuming a DMM reading of about 6 V on a lamp wire proves a fault - PWM dimming gives an average reading and needs a scope. Fitting higher-wattage bulbs that overheat the housing and wiring. Touching a halogen capsule. Diagnosis by swapping parts instead of checking supply and earth at the lamp under load. Ignoring a corroded earth that makes several lamps glow dimly or cross-feed through each other.
Rule of thumb. Both a feed fault and an earth fault can extinguish a lamp; load-test the earth with a known load before condemning the bulb.