Originally Posted by HeyYou

The PCM has no real way to tell if the data its getting from the O2's is correct or not. So long as the heater circuits are working, and the PCM sees voltage from the sensor switching hi/lo, it believes everything is good. Trouble is, while it LOOKS like it is working properly, the sensor could be misreading the exhaust, giving bad data to the PCM, and the PCM adjusts injector pulse width according to that bad data. That can give you a rich, or lean condition, (depending on which way it errors) and the engine just won't run right. Easy test is to unplug the front O2 sensor(s), (how many you have depends on year, and federal or cali emissions.) and go for a drive, if it runs better, replace the sensors.