From the Ultralyte user manual.
The JAM display indicator (
page 15) on the back panel screen may blink
during a measurement, and may be accompanied by the jam tone
(page 21). This indicates that the instrument is being flooded by light
and is having difficulty detecting its own signal. There are two possible
You are targeting a strong light source such as headlights.
A targeted vehicle is employing a laser jamming device.
Regardless of the level of interference, you will never get an erroneous
At a low level of interference, a speed may be captured or
an E03 error code may be displayed.
At a high level of interference, an E07 error code will be
Figure 17. Sample Speed Measurement - Departing Vehicle
The number code does not necessarily reveal the jamming source. The officer pretty much has to guess from this point forward. If you can turn it off, let him get a shot and continue on your way.