Bad cat?
- Thread starter clarknm
- Start date
I am guessing that B1S1 means bank 1 pre cat and B1S2 means B1 post cat?
Having both bank 1 and bank 2 on the display makes things kind of hard to look at. That also looks like frame data rather than continuous voltages which can make the lines pretty jagged (like Doc's V tach - although a normal ECG would also look pretty bad for an O2 sensor). The up stream O2 sensors should fluctuate above and below 0.5 volts fairly quickly and it sort of looks like B1S1 and B2S2 (red and green) are doing that, although it almost appears that there is a bit of an offset.
If this is at idle the cats can store enough oxygen in the catalyst to cause the downstream O2 sensor to read an almost flat value like the blue line for the bank 1 downstream O2 sensor. Depending on what the engine is doing the downstream O2 sensor may oscillate above and below the mid point; but, at a very slow rate. I would say that your bank 1 cat looks like it is working based upon what the bank 1 downstream sensor voltage is doing.
Your bank 2 downstream sensor voltage (orange) does not look so good. It sort of looks like it is oscillating at the same rate as the upstream sensor voltage (green) which suggests that the cat is having a reduced or no effect. It does look like it flat lines for a bit (running with Doc's metaphors) and then recovers. Not sure what is up with that. Anyway, looks dodgy. If you are having trouble getting your bank 2 cat monitors set a dead catalyst might be the problem. A savvy emissions tech with some experience can do a very simple trick by feeding some propane gas from a torch into the intake and then shutting the torch off and watching how the O2 sensor voltages swing from rich to lean as the ECU corrects. If the cat has lost its O2 storage capability it will show up in the way that the downstream O2 sensor voltage swings when the propane is switched off.
Summary; bank 2 cat looks dodgy and I recommend that you get a tech to do the propane test to confirm that you have a cat problem. The test would also show the response of the bank 1 cat.
Having both bank 1 and bank 2 on the display makes things kind of hard to look at. That also looks like frame data rather than continuous voltages which can make the lines pretty jagged (like Doc's V tach - although a normal ECG would also look pretty bad for an O2 sensor). The up stream O2 sensors should fluctuate above and below 0.5 volts fairly quickly and it sort of looks like B1S1 and B2S2 (red and green) are doing that, although it almost appears that there is a bit of an offset.
If this is at idle the cats can store enough oxygen in the catalyst to cause the downstream O2 sensor to read an almost flat value like the blue line for the bank 1 downstream O2 sensor. Depending on what the engine is doing the downstream O2 sensor may oscillate above and below the mid point; but, at a very slow rate. I would say that your bank 1 cat looks like it is working based upon what the bank 1 downstream sensor voltage is doing.
Your bank 2 downstream sensor voltage (orange) does not look so good. It sort of looks like it is oscillating at the same rate as the upstream sensor voltage (green) which suggests that the cat is having a reduced or no effect. It does look like it flat lines for a bit (running with Doc's metaphors) and then recovers. Not sure what is up with that. Anyway, looks dodgy. If you are having trouble getting your bank 2 cat monitors set a dead catalyst might be the problem. A savvy emissions tech with some experience can do a very simple trick by feeding some propane gas from a torch into the intake and then shutting the torch off and watching how the O2 sensor voltages swing from rich to lean as the ECU corrects. If the cat has lost its O2 storage capability it will show up in the way that the downstream O2 sensor voltage swings when the propane is switched off.
Summary; bank 2 cat looks dodgy and I recommend that you get a tech to do the propane test to confirm that you have a cat problem. The test would also show the response of the bank 1 cat.
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I was hoping to reformat but excel was unhappy with the time offsets for the data points. Over the whole log time both sensor 2 values seem to follow sensor 1 at times, bank 2 more so. I didn’t log anything else hoping to get higher sample rates on the o2, so I can’t say for sure what the engine load was at any point.
Thanks for the insights!
Thanks for the insights!
Depending on operating conditions, the downstream sensor (2) will transition from above 0.5 volts to below 0.5 volts. At the risk of making generalizations, it will do this much more slowly than the upstream sensor and under idle conditions may hang around 0.7 - 0.8 volts for long periods of time. At idle, if the rear O2 sensor voltage transitions quickly from high to low that is an indication that the cat is not storing O2. If you repeat those measurements at idle on a hot engine and the B2S2 voltages follow the B2S1 voltages like they do in the graph, that is suggestive of a cat problem.
My comment on the graph was more that if you only plotted the B2S1 and B2S2 voltages, it might be easier to discern the relationship between the two. Right now, it sort of looks like they are similar - which is not good.
This U tube video provides a relatively non technical discussion of evaluating the transition voltages on the rear sensor to diagnose the catalytic converter performance. He talks about the difference in transition times starting around 2 min 35 sec in the video.
Understanding & Live Graphing of the Rear O2 Sensor: OBD4Everyone Ep.19 - YouTube
If you want to be doubly sure that the cat has failed, the propane test is a good way to confirm that the cat has failed. It does require a tech who has some experience to interpret the results (a bit like a leak down test).
My comment on the graph was more that if you only plotted the B2S1 and B2S2 voltages, it might be easier to discern the relationship between the two. Right now, it sort of looks like they are similar - which is not good.
This U tube video provides a relatively non technical discussion of evaluating the transition voltages on the rear sensor to diagnose the catalytic converter performance. He talks about the difference in transition times starting around 2 min 35 sec in the video.
Understanding & Live Graphing of the Rear O2 Sensor: OBD4Everyone Ep.19 - YouTube
If you want to be doubly sure that the cat has failed, the propane test is a good way to confirm that the cat has failed. It does require a tech who has some experience to interpret the results (a bit like a leak down test).
So I replaced the cats and the car smells much better, but it still wouldn’t complete the test. The voltages looked better but nothing. So I broke down and took it to the dealer, they said the “oxygen sensors weren’t cycling properly”. They of course want to put factory sensors and cats in. I have Bosch in there now, so I just ordered a set of Denso.
