The California smog check does not inspect the a vehicle's brake light (brake system). Typically the brake light turns on if your vehicle is low on brake fluid or if any of the brake pads have worn out to unsafe limits.
Your vehicle will not fail the smog test if any of these lights are on... however if the smog technician feels either of these systems might cause a safety concern during the smog inspection, he/she may refuse to inspect your car.
