Hurray, Dash lights and codes!
 

So, I've got some dash lights...

Brake, Traction Control, Hill Descent, and ABS turn on at start, go off, then turn back on.

That said, Midas read some codes for me this afternoon. They came back with P1413 and P1590.

From Land Rover Discovery 2 P code list complete with transmission fault codes it looks like they're P1413 - Secondary air system fault bank 1 Drive cycle D:Signal out of range - above maximum and P1590 - Rough road signal implausible Drive cycle A:Signal implausible.

The workshop manual says a P1413 indicates "Secondary Air Injection System – Air control valve always open Bank 1 LH (Excessive SAI flow during active leak test)".

The workshop manual states a P1590 code means the hardware is OK, but the SLABS ECU is reporting an error.

What would you all recommend for troubleshooting these issues?

How about doing a search on this site for the 3 Amigos, there are hundreds of threads that will answer your question, then if you still need help shout and be specific.

The brake, TC, HDC, and ABS lights on could be the three amigos. There are many posts about that which you can search, but i think if the brake light is also on, as in the parking brake light, you may just be low on fluid or have a bad parking brake sensor. I can't help with the SAI.

If you were low on brake fluid and the 4th light were on, it would not cause the 3 amigos.

Thanks Spazmoid! The brake fluid level was the first thing I checked. It's right where it needs to be, and I reseated the connector on the cap during the check. I may siphon some fluid out, on the off chance it's some sort of an overpressure situation causing the Brake light to illuminate.

I was hoping someone might have more insight into what appear to be emissions and sensor based codes(assuming the codes are actually relevant to the issue). I've done quite a bit of lurking, but there doesn't seem to be anything addressing these codes specifically.

As for searching, and "three amigo" specifics... I've done some looking.
I'm not sure how much more specific I can be(short of throwing money/parts/labor at the issue to say "this didn't fix it so far"). Are you suggesting I follow the 50 step tear down of the ABS modulator? Would you recommend I see about renting or purchasing an RSW solutions Amigo ABS scanner? If the answer most likely to fix it is "Follow the tear down", I'll do that. It's not clear from any of the searching I've done that this is the case.

Sure there is plenty of information on your one and only emission codes, it is a failing SAI valve which has nothing to do with your 3 amigos.
As for them, if you have read anything, you will see it is going to take a special scanner/computer with Rover software to read and clear the codes once the repairs are made. How mechanical are you?
As for the 1413 code, down load the Rave CD and it will walk you how to test and repair that code.

if you plan on keeping that truck you should invest in the amigo. it could be a hub, speed sensor, or the modulator as you mentioned. the $200 for the amigo will pay for itself. $150 for a dealer just to read it then another $150 to clear it after you fix it. The 1590 is a soft code shouldnt set off the CEL as often as the SAI code.

Would you recommend I see about renting or purchasing an RSW solutions Amigo ABS scanner?

yes

I would be willing to bet it is a wheel speed sensor, that was my problem when I had the three amigos, they run about $130

don't ever bet on the 3 amigos. you will always lose.

read the codes. fix that problem. can be hub, sensor, abs mod, or other things