Thread: Catalyst Monitor "Not Ready"

I don't know for certain what the problem is, but I would trace the wiring to the rear oxy sensor it goes down behind the alternator and into the shield above the cat if the sensor is not getting voltage to heat up to working temp it will give this code. It might have gotten too close to the exhaust and be burnt/broken or maybe the sensor is bad. I think the rear oxg sensor because if it was the front one it should be giving you bad gas milage. good luck

at 235k i would say the cat is dead or real weak, cats get HOT real hot when working properly the rear o2 sensors monitors the cat emission output if u look at it on a good labscope it would show a flat line waveform, where the front o2 shows a waveform that looks like a snake it does monitor the amount of oxygen coming out of the engine and adjust's the fuel richer or leaner depending on need. If the cat is not hot enough it may show a ripple in the flat line waveform causing the cat not ready, may not be enough to set a code, if the heater in the o2 were bad or the sensor was dead it should set a code. The cat and the o2 sensors usually dont make it that long without a problem, it will not cause a fuel mileage issue would not worry unless u need a emission test