tomw wrote:The O2 sensor may be failing to detect the actual exhaust gas reading, so before condemning the catalytic converter, I would check the O2 sensor for proper operation. A lot cheaper, and easier to do than replacing the manifold.
Well more than likely it's going to be a waste of money replacing it, see below:
slavrenz wrote:With all due respect, I'm not at all surprised that [replacing the rear O2 sensor] didn't work. For some reason, many people think that a bad rear O2 sensor can cause a low efficiency reading for the catalytic converter. A little understanding of OBDII theory goes a long way here. The O2 sensors monitor the oxygen content in the exhaust as compared to the outside air - in the exhaust before the catalytic converter, the oxygen content is rapidly changing because of unburned hydrocarbons, NOx, carbon monoxide, and other pollutants that are present in varying quantities; thus, the front O2 sensor fluctuates quite rapidly. Ideally, this exhaust then gets treated by the catalytic converter, which removes most of these pollutants and makes the oxygen level much more stable; as a result, the rear O2 sensor fluctuates very slowly when the converter is working properly. An efficiency threshold code gets set because the car sees both the front and rear O2 sensors as fluctuating at close to the same speed, implying that the catalytic converter is no longer doing a whole lot to stabilize the oxygen content in the exhaust and remove pollutants.
If you understand this, then it becomes clear that the rear O2 sensor should be one of the last things to check. If the rear sensor goes bad, it is going to typically make the sensor less responsive, which means that the voltage will fluctuate more slowly or erratically. If the sensor's fluctuation slows down, the car would actually interpret this as an increase in the converter's efficiency, since it interprets a slower fluctuation as a more stable oxygen content. If, on the other hand, the front O2 sensor were to go bad, it would slow down that sensor's fluctuation or make it more erratic as well. If this fluctuation rate gets sufficiently close to that of the rear O2 sensor, the converter efficiency code would trip, even if the converter was in fact properly functioining.
The only way that a converter efficiency code could be caused by a bad rear O2 sensor is if the sensor's voltage fluctuation somehow sped up relative to the front sensor. Since this is highly unlikely to occur due to a sensor malfunction, the rear O2 sensor will almost never be the cause of a converter efficiency code. This, of course, assumes that there are no codes set for the O2 sensors themselves - those should always be diagnosed first.
The point is that if you have a converter efficiency code pop up, I would start by replacing the front O2 sensor (there's a good chance that this will help bring back some of your vehicle's mpg's and performance anyways). If that fails, then there are only a limited number of other reasons for the code, and a bad rear O2 sensor is almost certainly not one of them (in the repair manual for my other car, testing the rear O2 sensor is not even listed as a step in the troubleshooting process when a converter efficiency code pops up). At that point, I would make sure that I didn't have any exhaust leaks in front of the converter (this could also throw off the oxygen readings and make the car think that the cat isn't working right), and if the car has sufficiently high mileage, I would go ahead and replace the cat (assuming you've corrected for any obvious reasons for failure, such as a misfire). If the car is less than 8 years old and has less than 80k miles, the dealer is obligated to fix your converter for free per a federal mandate through the EPA, so take your car to them and be done with it.
I hope this helps...
From here, the first link I mentioned: viewtopic.php?f=8&t=12242