Our ECU's are reflashable, unfortunately Toyota hasn't been very forthcoming with the details of the flashROM in the ECU, or the methods used to program them in the first place.
I know there are people out there who will contradict what I say in this post, they'll say stuff like "I know a toyota tech, and so-and-so says the ECU is not reflashable." This is totally untrue.
There are two types of ECU's nowadays:
1) the kind with either a multi-pin connection to connect a programmer, or those which can be reprogrammed through the ODB2 connector. This is typical of Fords mostly, which can accept a programmer or an aftermarket program on a chip straight from the factory
2) those ECU's with a removable FlashROM chip which must be removed from the ECU itself, and re-flashed trhough an external reprogrammer.
Our cars use the 2nd method, BTW.
All ODB2 computers will utilize a flashrom in their construction, this way, if there was a major screw up in the factory progam, and technicians catch it in the future, a fix is as simple as reflashing the rom with an updated program (this is also used in PC motherboards, modems, videocards... You name it, it's a standard nowadays when it comes to software upgrade paths)
It is entirely feasible to reprogram the factory ECU, only problem is Toyota isn't forthcoming with specs on the ECU or how it's programmed.
The flashrom is there to retain the "learned" information about your driving habits, changes to the engine, Intake
, exhaust A/F ratios, etc. This is why many piggybacks don't work well, since the ecu maps around them, and stores such changes in it's flashROM. Since there are certain aspects of the ECU that cannot be changed by default, a person with a flash programmer could theoretically change the ECU programming, and the ECU won't rewrite it, since it doesn't have the capacity to overwrite what it believes to be the stock parameters in the first place (even though you just overwrote them with a custom map)
Many companies, like Hypermotive, neuspeed and Diablosport have used this method before, but without knowing specifics about the ECU, it would be difficult for the inexperienced to reprogam it easily (hence why the PowerFC is so popular these days; it's ready to program from the get-go)
I have experience working with flashROM, I use it with old console systems to load a disk-based game ROM to specialized, rewritable cartridges to run software on the actual hardware for beta testing. Aside from that, I reprogrammed the ECU on my old Escort before, It was ODB1 (meaning, no actual Flashable rom, just a plain old flash-once ROM chip (not reprogrammable) So I needed to use an external, reprogramable plug-in ROM (which is sweet!)
Our ecu would need to be opened up (whic voids the warranty) the FlashROM chip removed from it's socket, plugged in to a PC-based reprogrammer (which you also need software for). You need to know the specifics of the sourcecode used on our ECU's to program them, and write a compatible ROM program, which is flashed onto the chip, which is then reinstalled in tthe socket on the ECU's board, and the enclosure is reassembled around the board and reinstalled. If the ROM is soldered onto the board (like on many VW's) you will need to de-solder the ROM and solder it back on after programming (which can be a pain if it's pad-soldered on there)
Most ECU's will use a chip socket, though, in case the ROM goes bad, it's easier to replace a chip than the entire ECU, makes it easier to rebuilt them too.
So, can it be done? the answer is yes.
Has it been done? the answer is yes.
Is it easily done by the home installer? the answer is possibly, it depends on if the installer has the needed hardware, software and know-how to pull it off.
If this sounds involved, don't be shocked, because it really is. Your best bet is to either go with a nice piggyback module like the Unichip, or with a new ECU like the PowerFC or the Brash Boy units. they are easier to reprogram, less involved labor-wise, and will probably cost less than buying a programmer box, all the software, and the computer to run it all. Plus, you need to be able to know how to decompile the factory ROM program and backwards-engineer it to write your own software for it.
Possibly worth the investment if you want to start a buisness reflashing ECU's, but not so much if you are a tuner looking just to readjust your maps yourself.
Brash Boy uses this method to reprogram the ECU, it's a factory ECU, with a different program on it's flashROM.