I'm looking at a 2003 Limited 4WD V6 with 185k. It's a solid, straight truck, in good condition, even the driver's side leather isn't cracked (but the dash is, lol). No leaks that I could find, no rust at all (yay Texas). It's at a small used car dealer, the price is right, but I can always bring them down more.

My concerns:

• The CEL light is on, so I checked for codes; p0136, p0430. This can be as minor as an O2 sensor, or could be a cat. If it's a cat... why? Which leads me to the next...
• The early 4.0s have me nervous already with their HG issues; if it's a cat, I'm wondering if it's from burning coolant. From cold start, it runs smooth, and quiet (some 4.0s seem a to have a bit of mechanical noise). For the few minutes that it ran, there was no smoke or steam from the tail pipe, the oil was clean of water (albeit a little dark, time for a change), and visually the coolant looked fine. But, I've experienced HG and cracks that exhibit symptoms only hot and after driving under certain conditions...
• The trouble with 4wd actuators in these concerns me... It shifts into 4Hi and 4Lo, although it took a couple of seconds for 4Hi. How likely is this to be an issue in the future, and is there any preventative action I can take make sure it continues to work? Also, is it possible to convert it to an old-school manual lever shifter like the 3rd gens, should it fail sometime in the future?

I'm still waiting to have HG issues on my 2005. I've owned it 17 years.

CEL light - Don't know about those codes.

Head gasket - This post may be helpful, starting from the bold "Keep Calm" heading (ignore the part at the top about lifetime coolant, poster confused it with the supposedly lifetime [but really not] WS transmission fluid).

Actuator - Be sure to check it in all modes, and if they all work now, you should be fine. The few seconds to engage 4 Hi is normal in my experience. The issue with them not working is when people buy a 4WD 4th Gen and seldom, if ever, engage the 4WD. The manual recommends engaging all modes monthly and there's a PSA reminder post in the forum to do it. You "exercise" the 4WD by 1) driving 10 or more miles in 4 Hi and 2) finding a suitable spot to engage all modes (4 Hi & 4 Lo with & without locker). Be sure to read the procedures that are in the manual and should also be on the reverse side of the driver's side visor.

Edit: Bluesky beat me to it

The best course of action (actually recommended by Toyota) is using 4H/4L/CDL at least monthly which keeps everything lubricated and operating. If the service history is unknown, I'd swap the diff and transfer case fluid ASAP, then use routinely.

On the manual conversion, definitely possible with an FJ transfer case, several have done it here.

Funny, I called the mechanic next door to the lot today to see if I could bring it over and have them put it on the rack. I mentioned that I was a little concerned about the HG problems, and he responded, "oh, so it's the 4.0 V6?"... didn't even hesitate. He was very familiar with it and cyl#6 and said that these will usually run rough briefly with a cold start if the HG is bad, as coolant will puddle in the cylinder over night. Said if it starts smooth cold, don't worry about it.

It seams like most of the actuator failures I'm reading about are regular users; worked fine until it didn't. Here in Texas, I'm betting it's been years, if not a decade+ since it was last put in 4WD until me; most drivers here don't even think about it, especially ladies.

I skimmed the thread above, looks like it might be a worst-case challenge being the V6 4spd... sure would like to have that lever though, maybe I should hold out for a V8

To clarify, I think the vast majority of failures are from lack of use (ie: your situation). Some were able to un-stick actuators through a variety of methods and, once working and used regularly, tend to have minimal problems.

If it is a CAT and you want that code gone prepare for a pricier fix than on a normal car. Going through a exhaust manifold/CAT replacement right now. Not cheap.

you got one of the good ones. I think '05 was the year that everything started getting better. '06 HG failures were pretty rare.