Fault code help P0491 P0492 Check engine
AudiWorld Senior Member
Joined: Dec 2000
Posts: 1,383
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Re: Fault code help P0491 P0492 Check engine
P0491
16875
Corrective action
- Check fuse for Secondary AIR Pump
P0492
16876
Secondary Air Injection System (Bank2)
Corrective action
- Check hoses for Secondary AIR Pump Relay
16875
Corrective action
- Check fuse for Secondary AIR Pump
P0492
16876
Secondary Air Injection System (Bank2)
Corrective action
- Check hoses for Secondary AIR Pump Relay
AudiWorld Senior Member
Joined: Jun 2002
Posts: 1,032
Likes: 0
This is the air pump which is located in the same box
as the air filter. Trace the vacuum hoses that come from this air pump and you will probably find one has become disconnected. Mine was disconnected at a vacuum switch, top of engine drivers side. These hoses are easily pulled off.
Mine too still comes on every now and then and needs to be reset. PITA kinda.
I think it ultimately is an indicator of somewhat weak or failing O2 sensors, but changing all four of those is a PITA and costly too.
HTH
Mine too still comes on every now and then and needs to be reset. PITA kinda.
I think it ultimately is an indicator of somewhat weak or failing O2 sensors, but changing all four of those is a PITA and costly too.
HTH
AudiWorld Uber User
Joined: Mar 2000
Posts: 46,833
Likes: 1
In the Bentley shop manual CD.
You can get it for cheaper than this, at places other than Bentley Publishers.<ul><li><a href="http://www.bentleypublishers.com/product.htm?code=AD24&subject=2">Official Factory Repair Manual on CD-ROM</a></li></ul>
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Joined: Apr 2011
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I had the code P0491 come up on my 03 a6 2.7t. I have been researching this subject for 3 days, today followed all the threads and tore my engine down looking for the 1 of many possible problems, I found my 40amp fuse burnt out in my ECU box.
I replaced the fuse and reset the the computer. I just did my first cold start test and no SAP noise from the pump or from the relay box. I'm guessing from all the reading I've been doing that my CEL will come back on at 60 Miles when my ECU resets. I'll have to wait and see, I've never done anything to an engine outside of the Army and with the 2.7T being my first real project I feel confident I can figure this out for everyone.
I cleaned my MAFS and completely cleaned my TB out. My gain in low end powers is remarkable, I highly recommend any of you doing this. It took my about 4 hours but I was taking my time and learning as I went just based off what I read from these forums. If I had to guess I would say I'll have to get a relay tomorrow and mabe do the pump rebuild. ANY advice will be well taken and tested in the field. Thank you
I replaced the fuse and reset the the computer. I just did my first cold start test and no SAP noise from the pump or from the relay box. I'm guessing from all the reading I've been doing that my CEL will come back on at 60 Miles when my ECU resets. I'll have to wait and see, I've never done anything to an engine outside of the Army and with the 2.7T being my first real project I feel confident I can figure this out for everyone.
I cleaned my MAFS and completely cleaned my TB out. My gain in low end powers is remarkable, I highly recommend any of you doing this. It took my about 4 hours but I was taking my time and learning as I went just based off what I read from these forums. If I had to guess I would say I'll have to get a relay tomorrow and mabe do the pump rebuild. ANY advice will be well taken and tested in the field. Thank you
AudiWorld Member
Joined: Jan 2004
Posts: 166
Likes: 0
2.7t CEL code P0491
Thanks for the useful tip. My vehicle had the SAIP replaced but it was still reporting the CEL with the P0491 error code. Also the loud roaring noise from the engine bay during cold start was still occurring. By following the recommendations on this thread I visually verified the 40A fuse and it is still good. But as for the relay I didn't know how to determine if it is in good working condition. I purchased a new relay from VW for $30 and installed it. Started it up the next morning and no more loud roaring noise. I think the failed relay prevented the new SAIP from operating which caused the occurrence of that abnormal noise, but what's making that noise? Anyhow, the new relay seems to have resolved this issue for me and probably didn't need the SAIP replaced.
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