I'm getting p1171, p1174, p1412, and p1415 on a 2003 Discovery that's new to me, but that had the head gaskets replaced along with new plugs and wires about 10,000 miles ago or so when it was owned by my brother.

Do I need new o2 sensors up front? What should I check?

Or do I need a new MAF sensor?

or SAI?

I'm searching through the forum but don't know where to begin with it.

I should also note that the truck runs completely fine.

Since the codes you have are for BOTH left/right and not just one side, I would not think that the O2 sensors are bad.

I don't have SAI in my truck, so I don't have a lot of experience there. If I recall correctly, SAI only runs for a little while right after the truck is started. (I think?)

Usually with codes like these, people suggest looking for vacuum leaks since that is a much cheaper route than replacing the MAF. Do you have an Ultra Gauge or something that would display your gas mileage? If you do have a vacuum leak your MPGs can drop noticeably. But the best way to find a vacuum leak is to listen for a hiss or do smoke test.

You'll probably want to get some opinions from people with more experience in the SAI area, but I've dealt with vacuum leaks and failing O2 sensors in my truck.

I have a scangauge and get around 15 mpg. The SAI seems to run fine upon starting the truck. What's the best way to look for leaks?

Here is a good example:
https://landroverforums.com/forum/di...s-cigar-54235/

I had just found that! Headed out to get the tubing now.