CH4 SensorCheck Sum/End Char Error On GastroCH4ECK

Why does my GastroCH4ECK have a red cross, a pink circle saying 'PUR03 (2)' and have a retry symbol?

Try pressing the retry button. If the error occurs more than 3 times, please restart the unit. If failure persists then please contact Bedfont or their distributor for support.

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-SUM