2004 Prius EV (Electric Vehicle) Button Installation Instructions

These instructions allow you to take advantage of the built-in feature of what is commonly called "EV (electric vehicle) Mode" of your 2004+ Prius. In Japan and European versions of the Toyota Prius the vehicle is sold with an EV Button pre-installed by Toyota. For some reason, in the North American version this button is left off (note: speculation focuses on possible EPA/NHTSB regulations that were unclear as to whether the option to use 2 different 'fuels' was legal. Also it has been speculated that the decision came from Toyota, but no clear statement, at this writing, has come from Toyota. The EV button MAY become a dealer option or as a standard feature on future model year Priuses).
However, the HV ECU is programmed for the EV button. Accessing this feature is a basically simple procedure. One simply needs to plug a wire into the proper plug on the ECU, attach that wire and a ground wire to a momentary switch and EV mode is available.
Unfortunately, accessing the proper pin to plug the wire into the ECU plug, accessing the ECU and threading the wire and finding a ground are a bit tricky. These instructions are designed to take you step by step through the procedure.
What the EV Button Does (and Doesn't Do):
Recall that the Prius cannot be plugged in. ALL power for the vehicle is derived from the ICE (internal combustion engine) by burning gasoline. Extra energy normally wasted in a non-hybrid vehicle is recaptured and stored in the HV (high-voltage) NiMH (nickel metal-hydride) battery, and at times of demand that electrical energy is used to power the Prius. Thus, putting the Prius in EV mode via an EV button will not give you free power or improve MPG. In fact, the owner's manuals of the European and Japanese Priuses with the EV button warn that use of the EV button could LOWER your MPG. The general feeling among the North American Prius community is that there are times where using the EV button could, possibly, improve fuel efficiency. I'll try to summarize those situations at the end of the install instructions.
When you power on your Prius after it's been off for a period of time, it will briefly start in electric-only mode. After about 7 seconds, the ICE (internal combustion engine) will start automatically in order to warm the catalytic converter and the ICE itself. This is the period of time where emissions are the greatest in this otherwise very low-emission vehicle. If you had planned to simply move your car a short distance, you could avoid creating those increased emissions by utilizing the EV button.

Source