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Rincewind
Newbie


Joined: 10 Aug 2018
Posts: 20
Location: Cambridgeshire


PostPosted: Tue Apr 30, 2019 4:10 pm    Post subject: Post cat lambda voltage Reply with quote

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!!
 
  
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deMort
Long Beach


Joined: 21 Mar 2015
Posts: 6681
Location: Brighton


PostPosted: Tue Apr 30, 2019 7:34 pm    Post subject: Reply with quote

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 .
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Rincewind
Newbie


Joined: 10 Aug 2018
Posts: 20
Location: Cambridgeshire


PostPosted: Tue May 07, 2019 6:26 pm    Post subject: Reply with quote

Hi Demort,

The code reappeared today, It is P0136 bank 1 sensor 2 voltage low.

Here are some screenshots that I hope are useful
[img]









[/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...
 
  
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deMort
Long Beach


Joined: 21 Mar 2015
Posts: 6681
Location: Brighton


PostPosted: Tue May 07, 2019 7:10 pm    Post subject: Reply with quote

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 Smile

A poorly working cat will tend to give a high reading .. rich or a switching reading .. high / low etc etc .
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Rincewind
Newbie


Joined: 10 Aug 2018
Posts: 20
Location: Cambridgeshire


PostPosted: Tue May 07, 2019 8:58 pm    Post subject: Reply with quote

deMort,

You are a gentleman and a star. I will have a good check over and get the sensors replaced.

Cheers thumbsup
 
  
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