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Long Beach

Joined: 21 Mar 2015
Posts: 6652
Location: Brighton

PostPosted: Sun Jan 06, 2019 7:56 pm    Post subject: Fault code for post cat lambda sensor 986 Reply with quote

Ok ..

so a bit of a bitch job that taught me a thing or 2 hence im posting about it here .

Fault code for a post cat lambda sensor ageing fault .. i think it was P1096.

Code says .. fault with the lambda sensor so replace ... there is nothing else .. its old and not switching fast enough .

Basically these sensors get old and dont switch as fast as they used to , as this is save the planet stuff it generates and eml light on .. the car is not really effected running wise but an eml always panics owners .

The car takes days / weeks to monitor these systems so all you can do is replace it and send it on its merry way.

9/10 job done and move on .. not this bloody time though !

Car came back 2 weeks later with the same fault ..

Ok now i already know this is going to be a problem car .. you get them now and then .. it cant just be the normal items its going to fight you every inch of the way it can .. sigh ..

I scope / graph out the post cat sensors and i have a lean reading on this faulty side .. so thats what the car is seeing but reporting an ageing fault .

First thought .. is the cat allowing air to enter and giveing the sensor a lean reading .. car running and i feel for exhaust blow .. nothing ... small chuff at the rear joints but to be fair theres more of a leak on the good side than there is on the bad one .

Soooo .. im thinking wiring resistance , im thinking software .. im thinking is the actual bank on that side running lean .. im also thinking is there a fault with the other bank but the car is reacting to this side.

Basically where do you even start on this .

Wiring .. i check the wires from the sensors to the dme on both sides .. same or close enough resistance so its not that .

I reflash the dme .. now this is kinda scary .. if it goes wrong then its over £1k for a new dme .. but i have to do it ... its not that .

i smoke test the inlet mainifold .. small leak on the faulty bank .. not conviced but i fix it .. small leaks are not uncommon .. these aint new and the car will adapt reguardless .

Its not that.

Co check in front of the cats both sides .. they seem the same .. hmmm Question

Is there a fault on the other side .. hmm .. i try to swop the sensors side to side .. cant get the dam thing out ..

Ok banging head against the wall by now .. i keep coming back to is air entering the cat .. if its not running lean which the co readings pre cat would indicate then im kinda running out of ideas .

Final desperate attempt .. and i say that openly .. i jammed the smoke machine up the exhaust pipe .. filled the exhaust with as many rags as i could and let it run .. 3 mins later with smoke all over the place .. (i was never going to seal it so did what i could ) ..

Smoke wafting out under the cat heat shield .. not much .. but it doesnt take much .. a hair line crack in the cat and under a heat shield (fixed and not removable ) was the fault !!

Bolts will snap on trying to remove the mainifold / cat but thats the fault .. i know that before even fixing it Very Happy

Plug a tester in and it tells me whats wrong so i just follow the instructions .. hmm .. i think not !

On the plus side .. i now know a quick test when i get these codes to prove its the sensor .. downside was the time waisted which we cant charge for to find this fault .

Anyways .. a bit of info for you all .... never assume its just a sensor because thats what the fault code says .. sometimes the car is just going to be a bitch


My Daughter's Crowdfunding page Here


7pm - 9pm

Now At An Indy.
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Joined: 10 Dec 2013
Posts: 2791
Location: Berkshire

2012 Porsche 991 Carrera

PostPosted: Mon Jan 07, 2019 6:19 pm    Post subject: Reply with quote

Years of knowledge building, experience and skill got you out of that one ...must be very satisfying Thumb
991.1 C2, 986 Boxster S
Past pleasures: 997.1 GT3, 997.2 C4S Cab, 993 C2, 997.1 C2S, 996.1 C4
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Long Beach

Joined: 21 Mar 2015
Posts: 6652
Location: Brighton

PostPosted: Mon Jan 07, 2019 6:40 pm    Post subject: Reply with quote

I got the cat off today and cut away the heat sheild over it .. a tiny hairline crack.

Excellent ill weld it up .. welded then smoke tested the exhaust on the floor .. ok welded it again .. smoke test .. ok its opened up at another seam .. welded that .. yet another smoke test .. another pin prick hole appeared ..

Stuff this , customer is buying a new one as ive had enough of this PC .

Anyways .. the fault code for a post cat lambda sensor is not always the lambda sensor ....

I new i would get there in the end Very Happy

My Daughter's Crowdfunding page Here


7pm - 9pm

Now At An Indy.
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Joined: 13 Nov 2012
Posts: 688
Location: S.London

PostPosted: Tue Jan 08, 2019 9:49 am    Post subject: Reply with quote


That would have had me stumped! I had enough issue with the code I had which ended up being a broken wire, found thanks to you!

I'm looking at another Boxster which has a P0430 code, had new cats and o2's, so if I get it, this will be fun. not. Hopefully not something like your crack issue, I'd never find that.
Current fleet
1991 964 Carrera
2005 997 Carrera S
2008 957 Cayenne GTS
2003 Boxster 2.7
1994 Eunos Roadster S Special Turbo 270bhp!
1991 Mercedes 500SL
1979 Mercedes 280e
1995 VW Golf VR6 3.0
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Long Beach

Joined: 21 Mar 2015
Posts: 6652
Location: Brighton

PostPosted: Tue Jan 08, 2019 8:33 pm    Post subject: Reply with quote

I ended up fitting a new cat to the car as i just couldnt weld all the holes that kept appearing .

i scoped it today .. the other bank is operateing way behind this one .. no real supprise a new cat plus new lambda sensor .. im now worryed we may get the same fault but on the other bank in about 10 days time .. sigh

Your code indicates an ineffective or dead Cat ... new cats and sensors and it will be fun !

Wiring check for resistance from sensors to dme is the first check , second would be a reflash of the dme .. reprogram it basically .

Another possibility .. there is actually a running fault but for what ever reason has not triggered a fault code .. a few actual values i would like to look at with that fault and that work done to be fair Very Happy

You can find any fault .. it just takes time ..

Thats what i keep telling myself Floor

My Daughter's Crowdfunding page Here


7pm - 9pm

Now At An Indy.
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Joined: 01 Sep 2018
Posts: 55
Location: Kettering, UK

PostPosted: Wed Jan 09, 2019 12:40 pm    Post subject: Reply with quote

Who'd want to work on old cars? Oh that'd be those of us who tinker with our older 986s. It is a PITA to do loads of work and not solve a problem but kind of rewarding when you finally get there and have learned something into the bargain.

Thanks for the write up deMort!. I share your pain though.
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