There is a glitch in the program of the computer to control the engine, and therefore, the warning light will not illuminate even if the rear oxygen sensor that secondarily controls air/fuel ratio breaks. Also, the warning light will not illuminate if the elements of the rear oxygen sensor are damaged, which can happen due to the effect of the ignition noise from the ignition coil. If the vehicle continues to be driven in these conditions, the secondary controls of air/fuel ratio may not work properly, causing the emissions to exceed the regulatory standards.