It took me a while the first time I had to change my oil in my 2004.
Try this:
1. Begin with power off.
2. Press the "Power" Button while pressing down the brake petal, (Prius goes to "Ready" mode.
3. Press the "ODO/Trip" button and select "ODO"on the display.
I still have to look it up every time I need to reset it.
4. Press the "POWER" button to turn the car off.
5. Press and Hold the brake petal with your foot.
6. Press and hold the "ODO/TRIP" button. Press the "POWER" button., (Prius goes to "Ready" mode.
7. Continue to hold the "ODO/TRIP" button, with in a few seconds the "ODO" display on the dash will flash "------" and then for 1 second the display will read "000000" then,the Maintenance Reminder Light will go out and the "ODO" now displays your correct odometer mileage.
8. Release the "ODO/TRIP" button.
9. The Maintenance Light is now Reset (off).
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