Sole owner of 2002--quirks to expect

Years of heat and UV can take a toll on those plastics, making them brittle if you ever need to remove them (say for replacing instrument panel light bulbs, which also could be burning out around this age). I shot a video with advice on getting the center finish panel out in one piece.

That indicates a failing torque sensor inside the steering gear. It mostly leads to symptoms like you've described, or moments where the steering wheel will shake like a wet dog. A very few people have reported that it actually powerfully spun the wheel on them in one direction or the other. That should be extremely rare, because the torque sensor has two independent elements mounted in opposite directions, so even when they are both electrically noisy, they normally won't agree on a direction, so the steering computer knows it is getting bad data, and won't act on it.

There was an extended warranty on the part; mine got replaced for free around 194,000 miles. But that's expired now, leaving the intrepid DIYer to potentially digging out the torque sensor for cleaning.

The Gen 1 has other sensors that are built the same way and cause similar nuisances when in need of cleaning. The accelerator pedal sensor will create the "Big Hand Syndrome" (feels like a big hand reached down from the sky and slowed your car down). The position sensors on the heat-A/C doors will lead to a sound like mice scurrying back and forth under the dash. Those are easy to clean. The accelerator pedal, not so much (the sensor case is glued together).

Ideally, you're inspecting those regularly, though. I do at each tire rotation. Often no new parts are needed, but you can catch developing problems in the bud that, if not caught, could lead you to needing repairs tens of thousands of miles early.

Toyota sold Gen 1s with two different sizes of aux battery: an original one (GS Yuasa) of less than 30 Ah capacity, and a later, somewhat larger Panasonic, 45 Ah capacity (later further improved to 54 Ah). The larger battery also had larger terminal posts and required different clamps on the battery cables. A retrofit kit was available from Toyota to change out the battery tray and clamps in the cars that came with the smaller one originally.

When replacing, it's good to know which one is in there.

 

No reason to pay $200 for a 12V battery! Convert the terminal clamps to standard American size, and any 51R case-size battery will do, and there is a wide range of quality and price in that category. I currently have an Interstate installed, going on four years old. Given the sheltered location, the 12V should last at least five years, and typically much longer.

2002 Classic owner here. Just turned 150K miles. Keeping it forever!

 

Lepidopteress, congrats to you on keeping your Prius' longevity!

Please invest in a code reader; even the cheap ones will give you a reasonable starting idea of where to look for a MIL/Check Engine problem, although code readers which will get deep into the weeds of subcodes (particularly for the Gen I) are very hard to find.

I suspect that at some point, you will encounter P0420 and the Check Engine light will come on. That means the catalytic converter has reached end of life. You might extend its life by periodic use of a product called Cataclean, but I can't speak definitively. Being in NM, start looking around for an exact-fit replacement cat (Eastern Catalytic #40544 is one) and a mechanic to install it for you, unless you enjoy spending a half day under the car doing it yourself.

Also, the HCAC mechanism after the cat converter periodically may need some lubrication with a high temperature grease. The HCAC mechanism is vacuum actuated, and so the short stretch of 1/4 inch rubber hose under the car from valve to the HCAC may start to dry out and give you grief.

I'm in the Northeast rust belt, and so I have found that the gas fill pipe (that the fuel cap screws into) gets corroded and then won't hold a vacuum, which gives rise to Check Engine and P0440. Sanding away the corrosion and smoothing the fill pipe, as well as removing any rust which may get embedded in the rubber gasket of the fuel cap, solves that issue. Give your fill port an inspection; it may not be an issue, given your location.

But other than that, you seem on top of things. Best wishes for many happy miles may you and your Prius enjoy ahead!

 

Oh right, I had forgotten that detail. Advised by PriusChat, I had slid under there preemptively and worked that valve back and forth while applying some anti-seize at its shaft. Never gave me any trouble. I am not certain that recovery is as easy if one waits for signs of trouble first.