OK, I did a search on this - found a bunch of issues with the X300, but nothing for the X350.

P0411 Secondary Air Injection System Incorrect Flow Detected

My 2007 XJ8 occasionally triggers the CIL and throws the P0411 code (along with lots of misfire codes), but it clears up quickly. When this happens, the engine idles real rough and often dies right after start, so something is definitely going on besides just the CIL.

When I first bought the car a year ago with 48K on it, this happened within the first week. I took it in for warranty work and the dealer couldn't find anything wrong - said I was buying cheap gas. Over the past year/10,000 miles it has happened maybe 3 more times, and the problem never lasts more than a couple of hours. Last time was yesterday - CIL came on at first cold start in the afternoon when my wife left work. This was combined with the usual rough idle. She drove about 30 miles without problems, but after a short stop, the car still had the rough idle and initial stall after restart. I checked the car this morning, and all starts and idles fine. Checked the codes, saw the P0411 and all the misfire codes, cleared them and took the car out for check ride - no problems, and no codes.

I can't fix it if it does not stay bad! Just wondering if anyone has seen this issue and found a cause on the X350? Thanx

 

Did you ever find the cause of your CEL P0411 code? When I clear the code on my '08 Vanden Plas it stays off, sometimes for a month, sometimes for a few days, but it keeps coming back. The car runs fine, no rough idle, etc. If this code is because the secondary air injection pump is not working then I don't particularly care, because if I understand that emission control feature it only comes on for a minute or two at a cold start to push extra air into the exhaust gases to help the catalytic converter burn off unspent fuel. It doesn't run again until the next cold start and has no effect on anything else. The pump itself costs something like $500 and it looks like you have to drop the front bumper to get to it, so lots of labor costs as well.

 

I'd be interested in learning more about the P0411 and P0414. I had both codes appear on my 2006 NA x350 and the engine would stall immediately after starting. Right now only for P0414 is logged. Like frcraig said, the secondary air pump is behind the bumper. If your air dam is removed you can reach up under the passenger side of the bumper about 3 inches from the reflector right below the outer headlight is the air pump and it's mounted on rubber grommets for vibration/noise insulation so it's actually difficult to hear. On my 06 x350 with the air dam removed I can feel the blower operate and can barely hear it when it cycles on for 35 seconds immediately when the engine starts and turns off when high idle cycles to normal idle ( after about 35 seconds ). Note: ambient temps in the morning at 80 F. If you have the air dam still attached to the bottom of the bumper you could probably use something like a stethoscope placed on the bumper just under right bumper reflector and hear the blower that way. Like frcraig said, it only cycles on at start up and the amount of time it remains on I think can vary depending on ambient temperature.
I've replaced vacuum switching solenoid XR855179 and the diverter valve C2C32933, the short rubber vacuum hose XR849100 from the switching solenoid to the diverter valve and the hard small diameter vacuum line C2C22716 that feeds the switching solenoid. I reset the DTC's and the P0414 re-appears after startup along with CEL.
The x350 Workshop US005 Electrical publications does not include the secondary air injection system since that system apparently was introduced after 2004 so I can't find much information on how to diagnose. From what I've learned the ECM sends a signal simultaneously to both the switching solenoid and the blower motor relay to cycle both on at once. The idea being the vacuum reservoir, which is where the hard vacuum line eventually mates above the right shock tower, ensures enough vacuum is available to the switching solenoid at startup and then once the engine is running the vacuum remains more constant. The switching solenoid picks and puts vacuum at the diverter valve which operates a diaphragm allowing forced air from the blower into the right and left exhaust manifold. When some criteria is met, which seems to coincide with high to low idle cycle about 35 seconds after startup, the blower motor cycles off and the switching solenoid closes which drops vacuum to the diverter valve which in turn closes the heavy diaphragm in the diverter valve to keep exhaust gases from reaching the blower.
There are two heavy gauge wires that feed the blower relay ( which is mounted on the bottom of the blower ) and then two smaller gauge wires ( red w/white stripe and green w/white stripe ) that feed the switching solenoid.
I don't know how the system detects the incorrect flow P0411 code. Is the measuring air flow in the pipes from the diverter valve leading to the exhaust manifolds? Or is it measuring current draw on the blower motor and posting that code if low current flow is measured? I don't see any sensors or wires to anything that resembles a sensor for anything but the switching solenoid and the blower motor. Sure would be helpful to find the supplemental material for the x350 secondary air injection system including wiring diagram and overall system description. Although I didn't offer you any solutions here, I wanted to provide the information that I pickup up while trying to ferret out the P0411 and P0414 code on my 06 x350.
Bob
For completeness, adding the description for P0414 - Secondary Air Injection System Switching Valve A Circuit Shorted

 

Test the 5 Amp F27 fuse in your front power distribution box. This fuse protects the Secondary AIR switching valve. I logged P0411 and P0414 codes prior to replacing all the components outlined above and P0414 remained. Turns out that F27 5 AMP fuse was open. No more P0411 nor P0414. My theory is that the switching solenoid was bad resulting in blown F27 5 AMP fuse. I included a photo from my 2006 x350 depicting the fuse.