Fig.1) Normal waveforms of upstream HO2S(the upper) and downstream HO2S(the lower) (chassis ground used)

Fig.2) Normal waveforms of upstream HO2S(the upper) and downstream HO2S(the lower) (sensor ground used)

Fig.3) Normal graph of upstream HO2S and downstream HO2S at idle.

After warming-up, if accelerator pedal is released suddenly around 4000 rpm, the HO2S signal reading will be lower than 200 mV resulting from fuel cut-off for the moment. Conversely, if accelerator pedal is suddenly depressed, HO2S signal reading will be around 0.6V ~1.0V. At idle, normal HO2S signal will switch from lean to rich at 3 Hz. With higher engine RPM the signal frequency will increase.