This symptom shows up in different ways:
The readout shows an upscale reading with no flame present:
This unexpected reading is usually indicative of electrical leakage within the burner assembly.
The analyzer displays a loss in sensitivity, i.e, the same concentration of test gas reports a lower measured concentration over time:
This is often indicative of regular system degradation over long periods of time; the fuel and air supply systems (filters, restrictors, etc) as well as the sample restrictor and capillary can develop clogging, and the electrical elements of the burner can be partially short circuited, by extended operation without cleaning and rebuilds on the pertinent systems.