With ignition on press grey and black button together till it beeps, switch ignition off (now in sleep mode)..... after a minute switch back on and alarm should sound press grey and alarm now switched off and re booted....good luck!
Always use grey button only to switch on and off.....
Mine did that when I first had mine , gave it a good wash and the alarm played up . Ended up lifting tank and drying / wd 40 in connectors under there . Not played up since