3 minutes after the heated front oxygen sensor engine control unit constantly measures the voltage signal of the front oxygen sensor at intervals of 0.5 seconds. If two cycles of driving voltage signal of the front oxygen sensor for 0.5 s is less than 50 mV, the computer shows the code R0131 and warning light turn on the engine management system. The output of this code indicates that the front oxygen sensor or control unit determines a strong lean mixture.