Do the post-cat O2 sensors impact air fuel mixture or is this only done by pre-cat O2's and MAF?

Thanks

 

It's the latter. The post-cat sensors are for monitoring the cat efficiency.

 

Yes. The post-cat sensors impact the fuel mixture.

The post-cat sensors are used to increase the precision of fueling that is being done and being monitored by DME using the pre-cat sensors voltages.

The pre-cat sensors are monitored to know the DME has control of the fueling as it constantly varies the fueling on either side of ideal -- more on what constitutes ideal below -- and thus sees the voltage levels swing from under 0.1v to over 0.7v.

Additionally the DME monitors the post cat sensors to see their voltage signals follow the variations in fuel mixture changes and that the voltage levels show the converters are processing the exhaust gases correctly.

If the post-cat sensors generate a reading that indicates exhaust gas processing is not being done efficiently the DME can vary the fueling in an attempt to bring the post-cat sensor readings back into an acceptable range.

If the adjustment goes to far or there are issues the CEL is turned on and the appropriate error code or codes recorded.

 

Macster, is this mechanism Porsche specific? I don't recall any generic literature on how O2 sensors work documenting it. Also, does it mean this mechanism only kicks in IF something is not working properly, right?

 

One of my sources of info is from Bosch which provides DMEs for a variety of car brands so I would think it is not Porsche specific.

The sensors and DME represent a closed system of control. The sensors are used to closely control fueling at all times, well at all times the sensors/converters are hot enough and when the driver is not making extreme torque demands of the engine. At this point sensor monitoring is suspended and the DME fuels the engine in open loop mode.

 

Thanks, Macster.

 

Thank you Macster. I suspected they were involved in the mix. I have a non responsive post cat sensor that plays havoc with each bank if I swap sides with it. I will update with result when the new O2 sensor is installed.

JR

 

Post cat O2 sensor, bank 2 was reading 0.000. Symptoms included strong fuel smell from exhaust.

Replaced post cat sensor, readings appear normal and rich running / smell is no longer.

Confirmed, post cat sensor does have affect on air/fuel mix.

Thanks!

 

Sorry to open up a zombie thread, but I'm bloody convinced this is the problem with my 955 Cayenne.

I've had multiple codes and replaced MAF, plugs, coil packs..... You name it and I've replaced it.

Popped an OBD Bluetooth reader and linked it to Torque app and it shows that O2 sensor 2 bank 2 is erratic - peak, trough, peak, trough like a zig zag or letter W again and again.

Got misfire in cylinders 2, 4 and 6 on 200 mile journey today, plus P0300, P1009 and P1155.

I'm convinced this O2 sensor is the culprit and am having it replaced on Tuesday. I'm hoping the misfire is this and not the dreaded timing chain.

 

Might also want to get your car scanned with a PIWIS you might get more detail if it doesn't fix the issue - generic OBDII apps are good for initial diagnostics but thats about as far as they usually go

 

I have got an iCarsoft Por II arriving this week which will give me more detailed info. Plus I am hoping it will help me to reset the PSM as it is incredibly sensitive when I first drive away. I have to switch PSM off for a few minutes then turn it back or or else the car will lose power like it is slipping on ice.

Anyway, I'll find out on Tuesday if this O2 sensor is the culprit.