After replacing the post-cat O2 sensor, which had 125K on it and was 12 yrs. old I still had the P0411 code. Good old fashioned troubleshooting revealed that I did have a vacuum problem. The DV had an internal leak of the diaphragm and the vacuum reservoir does not hold vacuum resulting in a very low vacuum pressure. I had a deer/vehicle issue with the drivers front fender recently which helped exacerbate the situation. I haven't had the opportunity to check out the reservoir, but pinching the line to it made the vacuum return to normal. About 24-26 in/Hg. It was 4 in/Hg before fixing the DV, 6 in/Hg after, which was still not enough for the combi-valve to operate. Further investigation uncovered the reservoir problem. The vacuum circuit for the DV and the Combi-valve is very small and any leak causes a large drop in vacuum. At this point, the ECU is happy, no CEL and the Measuring block 77 shows a decent value for the secondary air pump. 4.78 g/s vs .78 g/s. To be continued...

 

I'm curious to know how you performed these tests. I have not been able to find the culprit of my P0411. What is the "DV" and where is it located? How did you test the vacuum? I suspect I have a vacuum leak that I have not been able to find because I pulled my combi valve and it works properly, the SAI pump turns on during cold start and shuts off after about 1 minute. I replaced some of the vacuum hoses already that did not pass visual inspection, but there very well could be other leaks that I am not aware of.

 

I thought I would post an update. I ended up taking my car to a shop that looked at my SAI system issue. They found that the SAI relay that is located under the ECU was bad. They replaced it and cleared the code. Only time will tell if that was the only issue, but looks good so far. They said the relay passed a preliminary test, but upon further investigation one of the terminals was bad.