2002 Volkswagen Passat I'm trying to figure out if my rotors needs to be changed out
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Drive the vehicle and do both light braking from 25mph then hard braking from 55mph, if the brake pedal pulses up and down or the steering wheel shakes or vibrates, then you have at least one warped rotor. If stops are smooth and relatively noise free then go to #2 below.
Pull the rotors and do a visual inspection, look for cracks (even small ones) and grooves, the rotor face should be smooth and flat. If there are cracks discard it. If there are small grooves or slight warping you can take them to a automotive machine shop or parts store and have them measured and resurfaced on a brake rotor machine.
Skip 1 and 2, just pull rotors off and take them to a professional with a brake machine, they can measure them for minimum thickness with a rotor micrometer, then spin them on the brake lathe to check for warp, then they can advise you if they are serviceable or not. *1
Some brake rotors are disposable, these are usually non vented (solid) rotors, they should Not be resurfaced and put back in service, if they are not warped or cracked and only have superficial grooves, you can just do what is call a pad slap.*2
*1 Not everyone that has a brake lathe or operates one is competent in measuring or making the decision on safe serviceability.
*2 Pad slap means just replace the brake pads and light sand the rotor surfaces to break the glaze. Pad slap method applies to vented rotors also..
All this being said, I personally do not re-use any rotors on passenger cars or light trucks, they are too cheap here in the USA to mess with machining them, and depending on who machines them (*1) what the results will be on your brake job.