I was idling in my driveway chatting on the phone when I noticed my CEL had just come on. My temperature was also almost at MAX. Suddenly, it dropped right back down to normal, but the CEL remains on. I am not loosing coolant, but the resevour was high.

I went to autozone and got the code pulled: P0483 Cooling fan rationality check malfunction.

I've checked all my fuses as good. I don't know how, or have no way to check the relays. The fans work, they come on when the AC is on.

Should I try replacing the thermostat? ANyone else solve this problem?

I'm still driving my 1998 Impreza RS, stock.

Sounds to me like a fan was off when it was supposed to be on (hence the code), then came on, returning your temp to normal. It's not the thermostat, it's something to do with the fan. Intermittent stuff is a pain. Maybe it's just a loose connection (check the wiring harness plug into your main fan [driver's side]; it should plug in on the side nearest the headlight). Another possibility is that the fan has essentially a "blind spot" where it won't start up if the rotor is at a certain point in its revolution; however, cooling fan relay is also a possibility, and a lot cheaper and easier to swap.
On my '02, the main relay holder is in the engine compartment aft of the battery, right along the driver's side at the edge of the compartment. It contains both relays and some fuses. On one end there is a single fuse for FWD operation. On the other end there are two fuses. On the FWD end, the two relays next to it are the two relays for the main radiator fan. Although the relay box should be in pretty much the same place on your car, I'd double-check the identities of the specific relays before going out and replacing them.
Good luck!

Yep, I have a very well marked fuse box in the engine compartment right where you said. I was at autozone today and tested every fuse that I could extract (I couldn't get 2 50As out, but they looked fine though the window).

Any idea how I can test a relay? It's been years since my electrical classes.

"Rationality check?" Holy cow. Now the damn codes are subjective.

"Rationality check?" Holy cow. Now the damn codes are subjective.
Yeah, my fan is being irrational I suppose.

It's just computerspeak for a check where they make sure something obvious is really true. For example, does 1 equal 1? If not, it means something is wrong with the assumptions; here, they were probably assuming the fan was turned on when the ECU told it to turn on.
Loki, since any problem you have is intermittent, you'd have to be able to test the relay right when it was malfunctioning, which doesn't seem to be very often. I'd just go ahead and replace the two relays for the main fan, since I would expect them to cost less than $10 each. Standard Bosch-type relays only cost a few dollars each online, so I wouldn't expect these to be ridiculously expensive (hope I'm right!).
If it happens again, quickly pop the hood and see if either of the 2 fans is turning (my bet is neither). You can also turn on the air conditioning, which should turn on the second (A/C) fan; since one fan w/the car stopped is better than none, even with the A/C load, that should get the water temp moving lower.

that happened to me too with the CEL. just clear the code and change your coolant. that should do the trick. it worked for me