My 2000 4Runner (4x4 california emission, 76K) recently had DTC PO420 poped up. I replaced front Air/Fuel sensor and cleared the code since then. However the same code appeared again and again. I figured the previous fix did not work. The rear O2 might be the real cause (I am so scared that I do not dare to think of the cat coverter!!!) Before I cough up another hundred bucks to buy a rear O2 sensor, can anybody tell me how can I know if a O2 sensor is bad or good? (The FSM states the method to check the resistance of the sensor, e.g. 0.8 -1.4 ohmm. However I measured it on a new front O2 sensor I got from sparkplugs.com, the value is 0?????????) Anybody fixed PO420 before?