Normal temperature variations can overstate your emissions. When the test box temperature was raised from 70F to 90F, the standard electrochemical sensor erroneously reported a concentration of 47 PPM of NO gas when no gas was present. This change is the result of the interaction between temperature and a sensor's history of exposure. The SEM sensor is temperature controlled per EPA's CTM-022 Sec 7.2.1 (Electronic manipulation of a sensor's output would understate emissions and is "unacceptable" per CTM-022 Sec 2.3.1)