2004 Toyota Prius EV (Electric Vehicle) Button Installation Instructions

What this is:
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.

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.

What You Need:
• Phillips Screwdriver
• Jeweler’s flat screwdriver
• 22 gauge dual strand wire (speaker wire works well)–about 6′ length
• Wire cutters/stripper
• Soldering Iron & Solder
• Drill & Bit (1/2″ if using the recommended button, otherwise determined by your selected button)
• Wire Coat Hanger–cut and straightened out
• Electrical Tape
• Paper Clip
• Momentary Switch (I recommend and used the Radio Shack SPST Momentary Pushbutton Switch


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