Post cat lambda voltage

My 996.2 C2 threw an EML light last week. I bought an OBD dongle and read the codes. I thought I had taken a screenshot of the code, but of course hadn't before I cleared the code and can't remember what code it was. It was cat/sensor related though. I monitored the oxygen sensor voltages. Both the pre-cat sensors are switching nicely between 0.1 and 0.7 V and look very similar to each other. However, the post-cat sensors are not showing the same. Bank 1 is pretty much flat on 0.1V and bank 2 is more variable and around 0.6V. I had a search but couldn't find any info on typical post cat voltages. Any thoughts on what the typical voltage is and whether the difference is an indication of a duff sensor or a duff cat? Car has done 93k and appears to be on the original cats. Thanks in advance!!

Off hand .. bank 1 cat has past its sell by date . If you drive the car for 3-5 days .. so upto 5 warm up cycles then the car will register the fault again .. emissions take several cycles until they can detect a fault . A normal faulty cat will have a switchable signal similar to the pre cats so we are not 100 % its a cat fault yet .. i would look at the fuel adaptations .. its possible bank 1 is running rich which you cant tell from the pre cat sensor switching but only from the adaption figures .. I'm afraid that after clearing a fault code in the dme these are reset so you are back to driving the car for a bit until the eml light is back on . Do that and repost please .. cat faults wont effect the engine unless its actually broken up and causing a restriction .. and then its a lack of power .

Hi Demort, The code reappeared today, It is P0136 bank 1 sensor 2 voltage low. Here are some screenshots that I hope are useful [/url] [/img] You can see that the sensor basically flatlines. Any advice on whether this looks like a sensor that is on the blink or the bank 1 cat is a goner would be appreciated. I already have a set of O2 sensors on the way anyway as I got some in the sale over the weekend. Now need to know if I need a new set of cats as well. Presumably if the cat is not doing it's job, I am going to struggle to get through the MOT at the end of the month. (Would be good to gauge how urgent this is!). Many thanks in advance...

The figures look very similar to a job i had not long ago which was a hairline crack in the exhaust .. it was drawing in air and giving a weak reading . I know on the one i did it wasn't blowing and i had no indication of a leak in the exhaust .. i finally found it using a smoke machine . you have new sensors coming .. its not beyond reason that its just a faulty lambda sensor so i would start with replacing just that . have a good look for exhaust leaks .. flange gasket / corroded studs is what i'm thinking here . Your reading seem to be on idle hence the above .. i would normally test by a hard rev .. look at the values and then on over run and the values then . if its just a faulty sensor then obviously that's going to be a lot cheaper than a new cat .. so cheapest first A poorly working cat will tend to give a high reading .. rich or a switching reading .. high / low etc etc .

deMort, You are a gentleman and a star. I will have a good check over and get the sensors replaced. Cheers :thumbs: