Interpreting O2 sensor readings from an OBD2 scanner
05-17-2017, 02:53 PM
#1
2nd Gear
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Interpreting O2 sensor readings from an OBD2 scanner
First, some background to the issue...
I recently got a check engine light and a stored P2096 code, which according to my OBD2 scanner and Torque app relates to 'post catalyst fuel trim system too lean bank1'. I cleared the code, but it reappeared about 150 miles later. During a recent visit to Jasmine Porschalink for a seperate issue, I had them check the code on their Durametric and it showed the fault as 'Post catalyst 02 sensor bank 1, ageing'. I am not experiencing and loss of performance or poor idle issues. Fuel consumption has not changed.
In an effort to diagnose the faulty component I have been monitoring the pre and post cat O2 sensors using the graph displays on the Torque OBD2 app, but I need some help understanding the readings. As I understand it, the pre cat voltages should swing between around 0.2v - 0.8v per second (mine appear to do that just fine) and the post cat sensors should output a pretty steadyish 0.7v. Here is where one of mine seem a bit weird. With the engine fully warmed up, Bank 2 post cat sensor outputs a steady 0.7v at idle, but bank 1 is giving unusual outputs. Sometimes it will hold at 0.7v for a minute or two, but then it will seemingly follow the pre cat sensor (not identically, but the voltage swings up and down in a similar fashion). It will then suddenly drop to a steady 0.1v for a minute or two, and so on. These different outputs appear to be random and don't occur in a set order or for a set time. In short, the output is very unpredictable!
I understand that if the post cat sensor follows the pre cat sensor, it is a very good indicator that the cat has failed, but surely if my cat had failed I would not be getting such fluctuating/random readings and it would never be outputting a 0.7v reading, as it sometimes does? If anyone can offer some help, it would be greatly appreciated!
I recently got a check engine light and a stored P2096 code, which according to my OBD2 scanner and Torque app relates to 'post catalyst fuel trim system too lean bank1'. I cleared the code, but it reappeared about 150 miles later. During a recent visit to Jasmine Porschalink for a seperate issue, I had them check the code on their Durametric and it showed the fault as 'Post catalyst 02 sensor bank 1, ageing'. I am not experiencing and loss of performance or poor idle issues. Fuel consumption has not changed.
In an effort to diagnose the faulty component I have been monitoring the pre and post cat O2 sensors using the graph displays on the Torque OBD2 app, but I need some help understanding the readings. As I understand it, the pre cat voltages should swing between around 0.2v - 0.8v per second (mine appear to do that just fine) and the post cat sensors should output a pretty steadyish 0.7v. Here is where one of mine seem a bit weird. With the engine fully warmed up, Bank 2 post cat sensor outputs a steady 0.7v at idle, but bank 1 is giving unusual outputs. Sometimes it will hold at 0.7v for a minute or two, but then it will seemingly follow the pre cat sensor (not identically, but the voltage swings up and down in a similar fashion). It will then suddenly drop to a steady 0.1v for a minute or two, and so on. These different outputs appear to be random and don't occur in a set order or for a set time. In short, the output is very unpredictable!
I understand that if the post cat sensor follows the pre cat sensor, it is a very good indicator that the cat has failed, but surely if my cat had failed I would not be getting such fluctuating/random readings and it would never be outputting a 0.7v reading, as it sometimes does? If anyone can offer some help, it would be greatly appreciated!
05-17-2017, 05:21 PM
#2
Race Director
First, some background to the issue...
I recently got a check engine light and a stored P2096 code, which according to my OBD2 scanner and Torque app relates to 'post catalyst fuel trim system too lean bank1'. I cleared the code, but it reappeared about 150 miles later. During a recent visit to Jasmine Porschalink for a seperate issue, I had them check the code on their Durametric and it showed the fault as 'Post catalyst 02 sensor bank 1, ageing'. I am not experiencing and loss of performance or poor idle issues. Fuel consumption has not changed.
In an effort to diagnose the faulty component I have been monitoring the pre and post cat O2 sensors using the graph displays on the Torque OBD2 app, but I need some help understanding the readings. As I understand it, the pre cat voltages should swing between around 0.2v - 0.8v per second (mine appear to do that just fine) and the post cat sensors should output a pretty steadyish 0.7v. Here is where one of mine seem a bit weird. With the engine fully warmed up, Bank 2 post cat sensor outputs a steady 0.7v at idle, but bank 1 is giving unusual outputs. Sometimes it will hold at 0.7v for a minute or two, but then it will seemingly follow the pre cat sensor (not identically, but the voltage swings up and down in a similar fashion). It will then suddenly drop to a steady 0.1v for a minute or two, and so on. These different outputs appear to be random and don't occur in a set order or for a set time. In short, the output is very unpredictable!
I understand that if the post cat sensor follows the pre cat sensor, it is a very good indicator that the cat has failed, but surely if my cat had failed I would not be getting such fluctuating/random readings and it would never be outputting a 0.7v reading, as it sometimes does? If anyone can offer some help, it would be greatly appreciated!
I recently got a check engine light and a stored P2096 code, which according to my OBD2 scanner and Torque app relates to 'post catalyst fuel trim system too lean bank1'. I cleared the code, but it reappeared about 150 miles later. During a recent visit to Jasmine Porschalink for a seperate issue, I had them check the code on their Durametric and it showed the fault as 'Post catalyst 02 sensor bank 1, ageing'. I am not experiencing and loss of performance or poor idle issues. Fuel consumption has not changed.
In an effort to diagnose the faulty component I have been monitoring the pre and post cat O2 sensors using the graph displays on the Torque OBD2 app, but I need some help understanding the readings. As I understand it, the pre cat voltages should swing between around 0.2v - 0.8v per second (mine appear to do that just fine) and the post cat sensors should output a pretty steadyish 0.7v. Here is where one of mine seem a bit weird. With the engine fully warmed up, Bank 2 post cat sensor outputs a steady 0.7v at idle, but bank 1 is giving unusual outputs. Sometimes it will hold at 0.7v for a minute or two, but then it will seemingly follow the pre cat sensor (not identically, but the voltage swings up and down in a similar fashion). It will then suddenly drop to a steady 0.1v for a minute or two, and so on. These different outputs appear to be random and don't occur in a set order or for a set time. In short, the output is very unpredictable!
I understand that if the post cat sensor follows the pre cat sensor, it is a very good indicator that the cat has failed, but surely if my cat had failed I would not be getting such fluctuating/random readings and it would never be outputting a 0.7v reading, as it sometimes does? If anyone can offer some help, it would be greatly appreciated!
The post converter sensors will read around 0.7V (which is an absence of oxygen in the exhaust gases leaving the converters) with a slight variation that is in some way in sync with the varying mixture and the voltage swings of the #1 sensors.
Pretty much what the DME wants to see is not too much oxygen making it through the converters. It wants to see extra oxygen allowed into the converters during the lean swing of the mixture but this oxygen consumed in the converter as it processes exhaust gases.
If the oxygen level stays high and as a result the #2 sensor reading drops that's a sign the converter can't store oxygen during the lean mixture time for use during the rich mixture time. That's a sign the converter is bad.
When my Boxster was occasionally manifesting a P0430 error code I observed the #2 sensor voltage reading low.
However, the reading was not consistently low. It would often rise and read about the same as the other side. But it was low way more often than the other side and clearly the converter was "bad".
(Actually, the problem was a loose converter brick that would occasionally get out of position and affect exhaust flow through the converter. The converter needs exhaust flow and the heat this brings with it and the heat arising from the exhaust gas processing to sustain the exhaust gas processing.)
There are ways to know the senor is bad. The heating element can fail. (This happened to a sensor in my Turbo.) Or the sensor is in some way bad. One way is the sensor doesn't manifest a suitable swing in voltage when presented with too much or not enough oxygen. Or the sensor reacts too slowly.
With my Actron OBD2 code reader/data viewer I can bring up a menu of DME supported O2 sensor tests and command the DME run these tests and view the results.
If one catches the sensor at the right time one can clearly determine the sensor is bad.
Actually, that the DME flagged the sensor is bad is pretty sufficient. It uses the same tests and has arrived at the conclusion the sensor is bad.
Anyhow, it reads like in the case of your car the flagged sensor is bad, due to "age".
05-17-2017, 08:19 PM
#3
Drifting
The sensors vary by model year and the generic Part Number Lists sometimes miss this. So use the Porsche part number to Interchange when selecting your new sensors(Probably Bosch ? ). It is good practice to replace all 4 at once.
Also consider replacing the coils ,plugs and spark plug tubes+seals if doubtful.
Check also to see if you have a pending SAI-related code. If you have, start another Thread on that issue so we can help diagnose it.I mention this because the SAI does a self-check while the engine is warm and on rare occasions this can produce odd lean conditions on one Bank.There are holes for the SAI air in each cylinder head and these can get plugged with carbon.
Also consider replacing the coils ,plugs and spark plug tubes+seals if doubtful.
Check also to see if you have a pending SAI-related code. If you have, start another Thread on that issue so we can help diagnose it.I mention this because the SAI does a self-check while the engine is warm and on rare occasions this can produce odd lean conditions on one Bank.There are holes for the SAI air in each cylinder head and these can get plugged with carbon.
05-18-2017, 09:49 PM
#5
Burning Brakes
I did one pre-cat sensor for aging, then the other pre-cat just for balance. But in the several hundred miles in between the changes including several 200 mile plus 65MPH plus trips, I never saw any odd behavior on mine. Bosch makes the Porsche part but do use one with pigtail already attached. Any muffler shop can do the change in 15 minutes and that includes cool down time waiting for safe working conditions. Bring the part and negotiate a price. Special wrench and room underneath to work and you can do it easily enough yourself. The reset can be done with almost any code reader, no need for a Porsche specific one.