2001 Warning Lights, cannot retrieve codes

If you really did have only the "check engine" light and master warming light ("exclamation point triangle") on, I'm surprised you did not get at least engine codes. Even the most generic reader should be able to show those.

If by 'check engine light' you mean any amount of the other warning lights, that might be not so surprising.

There are a lot of generic readers that can access and read no more than 2 or three of the available ECUs.

The key thing is to use a reader that is capable of reading all the ECUs. Only then will you get the full picture.

 

Search Techstream on here for more info on reading the codes and subcodes

 

In Gen 1, many of the ECUs in the car have a fallback ability to blink out their trouble codes on the dash lights, in case you aren't getting access with a code reader. Even in Gen 2 and Gen 3 some of the ECUs still have that ability, but many of them did in Gen 1. You connect pins 4 and 13 (CG and Tc) at the diagnostic port with a short skinny wire, turn the key to the ON notch, and watch the cacophony of blinking lights on the dash. (To count out the codes, you have to focus on one light at a time and tune out all the blinks from the others.)

That way you can at least find some of the codes you have. Disappointingly, the HV ECU and the battery ECU do not have code-blinking fallbacks, so a code reader is the only way of finding out what they've got to say.

Gen 1 was designed before CAN became the big successful car networking technology, and all the computers in a Gen 1 are connected by a real hodge-podge of networking technologies, sometimes even using different protocols or bit rates over the same wires. Some of them Techstream may only communicate with by relaying messages through others.

In some cases where lights are on but codes aren't read, there really is a communication error somewhere that is preventing the code reader from reaching the ECU that has the codes. In that case, the blink codes you get might include some communication error codes that might confirm it's that kind of problem. Then that would call for a look at the network diagrams in the repair manual and wiring diagram to see where the communication breakdown might be, depending on the available information of what things can't see what other things.

 

+1 for Techstream. You need a "mini-VCI" cable from Amazon for $30ish, some software, and a Windows PC or laptop. When I set myself up for Techstream a couple of years ago, this is what I did: Attempting to get our original 01 Prius back on the road | PriusChat
That setup was with 32-bit Windows; I have since set it up on 64-bit Windows but there are a couple of extra steps.

 

I have had the same problem in a non-automotive context: one blinkenlight that I needed to count, near some other blinkenlights that I needed to ignore. A relatively opaque piece of material that you can easily cut with scissors (a cereal box works great) is helpful to mask off the blinkenlights you don't care about.