1. Look at the beam path. A cone, a puddle, a piece of debris or a marshal standing in the wrong place will block it. This is the most common cause and the quickest to rule out.
  2. Check the emitter has power. The battery packs run 55 to 60 hours, which means the one that dies is the one nobody charged. Swap in a charged pack rather than testing the flat one.
  3. Check alignment. The emitter has a spotlight beam pattern to make this easier. Re-aim it at the sensor and watch for the beam to register.
  4. Check the height. If the stands have been knocked, the beam may be passing over or under the car.

If the system has an activity log, read it. A cable or wiring fault shows up as bad data on the line rather than as silence, which distinguishes a broken connection from a blocked beam.