bicycle computer showing incorrect/no speed

I recently had a sigma BC 1200 computer installed on my bike. I bought it
and paid for installation at a reputable bike shop. On the 15km ride home
everything appeared to work, but the next day I took my bike out for a short
ride and immediately noticed that the display was showing my speed at either
below my speed (half, a third etc) or at 0kmh. When the bike is stationary
I can spin the wheel and the speed will usually show what I'm guessing is
the correct speed. The next ride I was getting 0kmh most of the time, other
than right turns. I'm thinking that the right turns move the magnet
slightly closer.. Anyway in the manual it does say that the magnet should
be 5mm max. from the sensor/switch. The closest it reaches is about 8mm. Is
this the likely cause? I would usually trust the manual, but I'd also like
to think I can trust this bike shop.. What do you think? btw the computer
has not been exposed to any rain.

Thanks, Ted


.

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    ... On the 15km ride home everything appeared to work, but the next day I took my bike out for a short ride and immediately noticed that the display was showing my speed at either below my speed or at 0kmh. ... Anyway in the manual it does say that the magnet should be 5mm max. from the sensor/switch. ... I would usually trust the manual, but I'd also like to think I can trust this bike shop.. ...
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