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P0491 Fault Code

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Old 11-29-2007, 10:37 PM
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sullivmt
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Default P0491 Fault Code

Have a 2003 996 tt. The 0491 cel code has come up twice in the last two months. The first time I reset and cleared the cel . About 6weeks later(today), it came up again. The code says it's the secondary air system. Do I need to worry about this code or is this a 'have a look at the next service' item? Thanks for the help.
Mike
Old 11-30-2007, 12:24 AM
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Loren
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P0491 Secondary Air Injection System, Bank 1 - Below Limit
Possibly cause(s):
- Electrical fault in power supply or line between relay and secondary air injection pump
- Secondary air injection pump relay mechanically faulty
- Mechanical fault in electrical secondary air valve
- Pneumatic secondary air valve faulty/sluggish
- Electrical fault in secondary air injection pump
- Mechanical fault in secondary air injection pump or secondary air injection pump blocked
- Vacuum system leaking
- Air hose to secondary air injection pump slipped off or constricted
Old 12-01-2007, 10:06 PM
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sullivmt
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Loren -
Thanks for the possible issues -
Mike
Old 05-19-2008, 08:19 PM
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MitchB
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I am getting the 0491 and 0492 codes.

Shop tells me it needs a new pump, and will have to have the engine lowered to replace it. Over $3K total. I'd like a second opinion. Is it reachable to check for loose plug or hose(s) w/o lowering the motor?
Old 05-19-2008, 08:47 PM
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Loren
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Check these parts for broken/leaking hoses, stuck valves etc.

Last edited by Loren; 07-23-2013 at 07:56 PM.
Old 05-19-2008, 09:30 PM
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MitchB
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Thx Loren - that definitely helps. But are these readily accessable from under the hood? From under the car? That was the bulk of my query...
Old 05-19-2008, 11:26 PM
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Loren
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All are on top of the engine - but other things may be in the way.
Old 05-20-2008, 04:50 PM
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Chuck Jones
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Thanks Loren...that was a GREAT quick reference to DIY check points....I saved that one to my saved file.
Old 05-20-2008, 06:03 PM
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PorschePhD
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Check the line to the diverter valves. When this devlops a hole generally this is the code you will get.
Old 05-20-2008, 07:02 PM
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MitchB
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I'm going to check everything in there. I've got new billet DV valves, so I'll find that DIY you wrote up and change them out as well.
Hard to believe my pump failed at 19xxx miles...
Thx.
Old 05-22-2008, 11:52 PM
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Zippy
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Any luck?
Old 05-23-2008, 11:22 AM
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MitchB
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I haven't gotten into it yet.
In order to check all the lines, I'll have to remove a few things in the engine compartment. Want to give myself plenty of time, as I haven't messed around back there on this car much.
I did find the Sec Air Pump, ascertain that it does in fact work as advertised. Starts on start up, and shuts off after a minute or so. I'm fairly confident that it's a leak somewhere.
I do have new Div valves to install from UMW, so those will go in as well.
We'll see. I'll post my results.
Old 05-26-2008, 11:17 AM
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Originally Posted by MitchB
...I did find the Sec Air Pump...
I have been trying to locate this. Was told to check the tube coming from the bottom of it. In the diagram, "PUMP" isn't used. Is it part #6 or #7? I'm looking to fix the same codes.

Thanks, Mike
Old 06-02-2008, 03:55 PM
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MitchB
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It's #6 in Loren's diagram.
Doesn't look like the drawing though. It's right below the oil fill cap. You'll hear it (and can feel it running) on start up...
I swapped my Div valves this weekend, and cleared the codes. No codes yet after 4 - 5 starts with 3-4 hoours driving. Keeping my fingers crossed for now...

Edit: Think I've pretty much confirmed I had a bad DV. Blowing into the "big hole" of one of them leaks air out the small (vac) hole... $3K saved and now I know who NOT to go to for service...
Thx R-list!

Last edited by MitchB; 06-02-2008 at 04:45 PM.
Old 06-02-2008, 06:57 PM
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I can remove the pump in less than 45 minutes.. You do NOT need to drop the engine...

However, is telling you that is bending you over!!! Again the pump can be removed thru the engine compartment opening..

Glad you installed the valves..

I strongly recommend installing small tie wraps to all the small hose connections.


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