P0491 Seconday Air Injection fault?

Hi all C3 owners!

I have a 2004 Citroen C3 (1,4l petrol engine with 54kw (73hp).

On my engine, there is this seconday air injection system integrated which now pops the P0491 fault code. I read this code as "insufficient air flow" into the exhaust manifold at start up (cold start).

How do I know where the error is?
Possible causes, of what I have understood at least, might be:

- The one-way check valve (placed between the pump and the engine). Note: I think this is a non-vacuum variant, meaning the valve should open/close by the airpressure itself. I cannot see any vacuum lines on the valve, therefore this assumption.

- The exhaust manifold could be clogged?

- Faulty/broken O2 sensor (is it the pre or post O2 sensor that checks this air flow??

- Any fuse maybe?

I do not want to order all those parts to solve my problem so what is most likely to have broken? (My pump is working fine - it blows air at cold starts).

best regards,
garo

Hello garo

garo wrote: ↑Wed Jun 19, 2019 10:47 am - The one-way check valve (placed between the pump and the engine). Note: I think this is a non-vacuum variant, meaning the valve should open/close by the airpressure itself. I cannot see any vacuum lines on the valve, therefore this assumption.

Just look a bit more closely for an electrical plug or a vacuum pipe. Then you will know. It's not common to have things control themselves and also there to be no feedback to the ECU.

garo wrote: ↑Wed Jun 19, 2019 10:47 am - The exhaust manifold could be clogged?

Probably not, otherwise the engine wouldn't run.

garo wrote: ↑Wed Jun 19, 2019 10:47 am - Faulty/broken O2 sensor (is it the pre or post O2 sensor that checks this air flow??

No, the O2 sensors don't measure airflow. They also have their own codes to monitor their performance.

garo wrote: ↑Wed Jun 19, 2019 10:47 am- Any fuse maybe?

Unlikely, but it's no biggie to check fuses by removing them and checking with a meter. The C3 fuse boxes are here

garo wrote: ↑Wed Jun 19, 2019 10:47 am I do not want to order all those parts to solve my problem so what is most likely to have broken? (My pump is working fine - it blows air at cold starts).

Sure. It never pays to change parts on a whim.

If you have a low air flow, you need to know if the pump is delivering enough pressure as yours may be a bit low because it's worn. Not easy to measure or know what the flow should be.

You may have a leak of the pressure in the system that is supplied by the pump, so check for damaged pipes and missing clips.

You could have a blockage in the system and you can strip parts out to investigate.


The diagram for the secondary pump may help


You can see the relay and it's an electrical valve.

Hi!

Thanks for the replies!

Hm, if the valve is electrical... from where comes the electrical feed? The only electrical connection I can locate is the electrical connection to the air pump. And between the valve and the pump is only a plastic hose..? And since the valve is attached directly to the exhaust manifold with 2 screws.. there is not possibility it is electrically connected in any way. Or I am missing something?

The air pump is changed about 6 months ago so its a "new one", the air flow is very good.

I am 99,99% sure its a non-vacuum valve. Here is what it looks like:
https://i.ebayimg.com/images/i/17312358 ... -l1000.jpg
https://bartebben.com/dailySQL/Images/i ... 0cysjo.jpg

Nowhere I can find a vacuum line to it, why I assume its a pressure-activated one.

The relay you refer to in the picture.. where would I locate it?

best regards,
Garo

This is hot you attach images to the forum

garo wrote: ↑Thu Jun 20, 2019 8:43 pm there is not possibility it is electrically connected in any way. Or I am missing something?

Sorry, the connection I saw was for the pump. As you say, if there is no connection to the valve then it can't be ECU controlled, which is unusual for emission control on the C3.

Do you know how to test the valve?

It doesn't look like it will come apart and go back together


The diagram doesn't show the location of the relay but the extra relays are often hidden under the BSM module. BUT, if the pump runs, then the relay is going to be OK because it does look like its a relay for the pump.

Does the pipe make a good connection that doesn't leak? Soapy water may help identify a leaky joint that would be loosing pressure that would give a low voltage reading on a sensor.



This video sums up the failure points and the operation of a secondary air system which suggests a worn valve and a worn pump

Hi,

Thanks for the ideas and tips.

I am suspecting it must be the valve since I have now for 2 days started the engine while manually keeping the valve open (by pushing it from the top when the black cover of the valve is removed), thus allowing air into the manifold at startup. I have made at least 5 driving cycles during these 2 days (I believe the MIL turns on on the third or fourth cycle. All cold start cycles in this case ofcourse).

My conclusion is that the valve is not opening when pump is blowin air into it at start up.

I have tried to blow air into the valve with my mouth (while demounted from the manifold) and it does require some hard blowing to feel the air on the other side of the valve. What I will try to do is to detach the valve from the manifold and then make a cold start to check if air is flowing through the outlet of the valve. If not - then the valve is worn, not opening when airpressure is applied.

I will order a new valve and see if it helps.

THank you for replies.

Garo

garo wrote: ↑Thu Jun 20, 2019 9:22 pm
I have tried to blow air into the valve with my mouth (while demounted from the manifold) and it does require some hard blowing to feel the air on the other side of the valve.

When your new one arrives, try doing the same test with some 'hard blowing' and you may find the new one is so much easier to operate? - that's a guess

Maybe it's just a bit bunged-up with carbon, but you can't beat fitting a new part than trying to clean up and old one

garo wrote: ↑Thu Jun 20, 2019 9:22 pm I will order a new valve and see if it helps.

THank you for replies.

Garo

I would be interested to see if that clears the problem, please keep the topic updated

I will get back to see if the problem persits when checkvalve is changed.

best,
Garo