CH4 Sensor Sync Error On GastroCH4ECK

Why does my GastroCH4ECK show a red cross and a circles with a code inside it?

Try pressing the retry button. If the error occurs more than 3 times, please restart the unit. 

If the problem persists or has not been resolved by the information provided above, please contact Bedfont or your local distributor for more advice.

GastroCHECK-SYNC