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Seeking expertise from all vacuum Gurus - P0492 that (really) wants to

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P0492 that (really) wants to stay...


Over the last month I’ve been troubleshooting a [P0492] Secondary-air system bank 2] error, which triggers the CEL of my ’03 Boxster S (DME 7.8).


I’ve taken apart the entire vacuum system, replaced any/all components that were faulty/worn out, but the error keeps re-appearing every couple of days. Now, the car runs perfectly well, it’s just I don’t enjoy a CEL light in my dashboard…


With Durametric, I can validate:

  • Secondary air pump (also works during the first 90 seconds of a cold start)
  • Secondary air pump solenoid (I can hear it clicking)


Can this be a VarioRAM issue? I’m asking because VarioRAM is installed in bank 2 in a Boxster, whereas the secondary air pump is in bank 1 (not sure if the description means physical location in the car or not…)

  • Both are connected to the same vacuum system
  • I cannot trigger the solenoid using Durametric. I believe they call it [intake Pipe Swithover] – can somebody confirm if this is the right activation??
  • I’ve also confirmed the solenoid works OK on a test bench - but again, doesn't work with Durametric


CEL usually comes on if I’m just cruising (very rare) of after a cold start, during the first 5 minutes of driving.


What am I missing??

Edited by JuncoJones
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Have you checked if the vacuum reservoir is holding vacuum? It can have invisible cracks at its bottom. Did you also replace all the vacuum lines?


With Durametric, you can log the four O2 sensor voltage reading (pre-cat and post-cat) on both banks and make sure you see something like this during cold start:



  • Upvote 1
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Thanks Ahsai. yes, the vacuum reservoir is new, as it had developed a small crack at the bottom.

Pretty much all vacuum lines are new as well, and I've used permatex gasket sealant on each connection.


That's a very nice (and descriptive) graph, btw. it will certainly help in my troubleshooting. I'll measure all sensors tomorrow morning, with the car cold.

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Well, I couldn’t wait until tomorrow morning… so I did some tests on the O2 sensors tonight :)


Looking at the graph above, after the initial 90 second warming up period, pre-cat voltage should be osciIlating as per rich/lean conditions, and post-cat voltage should be steady around 0.7v... None of my post-cat sensors are running steady at 0.7v:

  • bank-1 is most of the time at 0.7v,  but drops from time to time (like a pre-cat).
  • Bank-2 is running 'pretty-much' like a pre-cat

So my guess is that both cats are NOT working as supposed. But then, why am I NOT getting a CEL about cat issues and I’m getting one about the secondary air pump bank 2?

Edited by JuncoJones
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That could be because the cats are not warmed up enough. If you drive the car to warm them up completely, at idle the postcat sensors should be more or less stable at around 0.7v If the postcat reading still keep bouncing and following the precat, then the cat is marginal but may not be at the level to trip the cat codes yet.

Anyway, the part that's relevat to your error code is steps 3&4 in the diagram where the sensors read 0 (reading the O2 injected by the SAI pump). Do your sensors track the same way to 0? The DME uses this to detect if the O2 injection is working.

Edited by Ahsai
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Excellent plots! A few things I observed:


1) Bank 1 sensors and cat seem to behave normally (no worries about those momentary dips on the postcat reading).


2) Bank 2 seems to be not getting enough air injected as evident by the fluctuating precat O2 reading during the time the pump was ON. That's also what tripped the P0492. I suspect either the SAI valve port/line to bank 2 is restricted. Need to check if Part #7 is plugged http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=986-97-04&section=108-06


3) After the postcat sensor became operational, the bank 2 reading does not look normal. Either the postcat sensor is bad or the cat is marginal.


It would be really useful if you can drive the car for 20min or so and then let it idle. You should see the precat sensors swing up and down every second or so and the postcat ones should read around 0.7v more or less steady. If you doubt the sensors, you can always swap them between banks and retest. 

Edited by Ahsai
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This morning after doing the cold-start readings I went ahead and swapped the O2 sensors between both banks.

These are the results at running-temperature, which certainly looks much better... Maybe it just required some TLC, the O2 sensors was one of the only parts of the car I've never touched before. Btw, it's a breeze to swap those in a Boxster...


And by the looks of it, the O2 sensor (now) in bank-1 is getting old, as per its constant fluctuations.

What I don't understand is why the resistance keeps going up and down on both banks.


And tomorrow I'll do another cold-start reading and will post the results.


Thx again, Ahsai.






Edited by JuncoJones
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Nice plots and it's good to see the postcat reading stabilizes in 0.6~0.7v, which is good.


I don't suspect your O2 sensors as all of them swing between 0.1 and 0.7 and more importantly you don't have any O2 sensor related code. I expect to see the exact same behavior of the yellow boxes in my diagram tomorrow despite the sensor swap (i.e., bank 2 postcat still fluctuates when pump is on). Look forward to seeing what you find tomorrow.

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And by the looks of it, the O2 sensor (now) in bank-1 is getting old, as per its constant fluctuations.

What I don't understand is why the resistance keeps going up and down on both banks.


I assume you meant the postcast O2. No, the now in bank 1 sensor is fluctuating only very little. The scale is blown up so it appears to fluctuate a lot.

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you were right on the spot, Ahsai!  The behaviour is still the same, it has to be a physical air blockage in bank-2, either in:

  • the air tube itself (connecting the shut-off valve to the block)
  • the air passage inside the block (connecting the air tube to the exhaust)


Well, I guess I'm the lucky individual that has to remove both intake mainfolds AGAIN... practice is certainly the path to perfection   :)



Are you aware if there is any diagram of the air passages in the block?




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One day I'll get one :) I need the rear seats so driving a 996 now. These are all great sports cars!

I think once you remove that metal tube (part #7), you can probably try to blow into the port on bank 2 where the tube connects to and feel for blockage. Or you can remove the bank 2 header first and check how the small sai ports look.

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Well, I’ve performed a series of analysis on the ‘highly-procedural’ work that needs to happen to [successfully] clean the two SAI bypass channels on my ’03 Boxster S:


I’ve identified a potential show-stopper, which is braking an exhaust bolt (or more than one, for that purpose), which will pretty much derail the entire project…


To help address this issue, I’ve created a phase-zero, and actually remove each and every intake bolts (one at the time) to add anti-seize to its thread with the exhaust @ running-temperature, so when you’re ready to perform the entire cleaning operation, your chances of success will be certainly higher. And if you happen to break a bolt, the car will be still operational by having all the remaining ones. This is will be the right time to add a BIG new task, which is getting those broken bolt(s) properly fixed!!


Also, as a follow-up to a suggestion by Ahsai to report my findings back to the group, I’ll write a DIY tutorial about my own experience in doing this type of repair...

Edited by JuncoJones
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  • 6 months later...

Glad you got it sorted! Thanks for keepimg your word and I'm sure the diy will help others.

It's interesting to see the clay like material clogging the SAI port, and only on bank 2. I wonder what that was.

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  • Admin

All flags are in Ready state... very cool!


and here's the BEFORE and AFTER graphs of the 02 sensors, also posted in the DIY guide.





Please keep your replies (on the same subject) to one thread only.

It makes it confusing or looks like two separate issues when you post multiple times. Thanks.

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