Not all electrochemical sensors are the same.

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)