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P0491 & P0492 CEL Codes

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Hi Guys,

I had to bring my ride in to fix a broken shift linkage problem from last weekend. It's fixed and not really a major issue thank god. However, I asked them to check the recurring P0491 & P0492 codes my car keeps throwing every 2-3 weeks... Well they called me today to tell me everything checks out and my cat and/or cats are probably starting to fail. I have done a lot of reading on this and it seems very rare that a cat would fail - correct? It's usually something like a valve, vacuum leak, secondary air pump failure etc... Cats are brutally expensive and I am starting to doubt my shop in all honesty... I have been bringing my cars here for a long time and everything has been fine with them. Do I need a second opinion? What is the likelihood their diagnosis is correct with my cats failing? My car is flashed, Fabspeed air intake, Agency Power DVs, and exhaust. Thanks for your input as always in advance!

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P0491 and P0492 say your secondary air injection is not working. Check to see if somone forgot to plug it back in.

P0491 Secondary Air Injection System, Bank 1 - Insufficient Flow

P0492 Secondary Air Injection System, Bank 2 - Insufficient Flow

Possible cause of fault

- 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

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Thanks for the reply! My Secondary Air Pump fires up at start. Not sure unfortunately how extensively they checked over the rest of the system and this is my problem as they are blaming the cats. What is the likelihood the cat(s) are failing?

Thanks again!

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What is the likelihood the cat(s) are failing?
Not with those fault codes.

Because the secondary air pump is running does not mean the air is getting to the cylinders.

Look at the possible cause list - this is a secondary air problem.

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I suspect it's a vacuum leak, likely caused by a failed Diverter Valve. That's what it was when I got those codes. When I took the DV's off, one of them, if you sucked on the top, would breathe freely. If you haven't done it before, swapping the DVs is a pain. Once you've done it, it's pretty easy.

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  • 3 years later...

Figured I would just revive an old post as I am getting the 0491/0492 code on the engine I just installed. I reset the code and it may show up again in 5 miles or it may be 200 miles. Either way, the readiness will not set and I cannot pass emissions. I was afraid that I left off a vacuum hose or something so I lowered the engine and checked all of the hoses that I could see/reach and checked the diverter valves. All seem in good order. Here is my question: Should I be able to hear and tell that the SAI Pump is running when I turn the key to the on position, or does it only run after start up? Once I start the engine, I cannot hear the pump activate? I can put my hand on it but it's hard to tell if it is running due to engine noise/vibration. I pulled the plug and the top on number 1 side is hot, but it is hot even with the key off?!? 

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Should be on only when engine is running. If you have Durametric, you can activate it even when engine is off. If not, you can use a mechanics stethoscope that has a long thin metal rod. Touch the pump with the rod and you should hear it running. Check after 2min (after cold start) and it should be off and you can tell the difference.

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When I installed the new engine, I visually inspected the hoses and fittings and opted for those that looked the best. I used the Diverter Valves that were on my original engine as they were billet units that were fairly new. I went this morning and purchased a mechanics stethoscope and can now confirm that the pump is working on cold start up. It ran for about a minute and then spun down.  Short of removing the engine completely, it is very difficult to check all of the connections. The car runs great but I cannot get my tag for what could be a loose or cracked vacuum hose!

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Do you have any scan tools that could read real-time O2 sensor reading? If you do, you can confirm fresh air is truly injected into the exhaust by the sensor reading. If you don't see actual air, that means the SAI air valve(s) are not working correctly. This was taken from my car (non-Turbo '03 C2) but you get the idea.



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