Resetting the ECU after a reported code?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pumpanddump, Oct 3, 2022.

  1. pumpanddump

    pumpanddump Junior Member

    Jun 29, 2017
    South Carolina
    2007 Prius
    Curious how smart the ECU is.
    ( I'm talking in general terms of the process - not for any specific error code thrown.)
    If I get a check engine light or I run techstream see some codes, perform the repair for the code.
    Shut the car down after testing out the repair. Everything seems fine.

    Do I run techstream now again to see if the code is still present or gone?
    Meaning the ECU should find the correction and remove the check engine light or error code. So the ECU is smart.

    Or must I reset the ECU by unplugging the Accessory battery after every repair, essentially wiping the previous codes and running techstream again to see if the codes come back?
  2. Tombukt2

    Tombukt2 Senior Member

    Nov 29, 2020
    Durham NC
    2009 Prius
    No in tec stream you reset the light. I think I usually just run the diagnostic mode after I've done the repair to make sure the code is gone and for a lot of these things the light will just go off or not be present when I turn on the car and run tec. If it is then I push that little thing for DTC down in the corner and clear all the codes and then I watch the light turn off all by itself right in front of me and they'll be no codes It will show me that I think at the end of the light clearing I can't remember I use it so rarely
  3. ChapmanF

    ChapmanF Senior Member

    Mar 30, 2008
    Indiana, USA
    2010 Prius
    The exact details vary between Prius generations, and the repair manual explains them.

    For Gen 2

    In a Gen 2, there can be pending codes and current codes. If a code is defined with "2-trip detection logic", then the first time the ECU detects that issue, it sets a pending code, and it isn't a current code until the issue is detected a second time. The check-engine light only lights when it becomes a current code. If you then fix the issue and make 3 consecutive trips without it, the check-engine light will go out, but the code is still stored. The clear operation in Techstream, or disconnecting the battery, will make the code go away.

    For Gen 3

    I'll describe Gen 3 here also, just to show how it's different. Gen 3 has pending codes, current codes, and permanent codes.

    Pending and current work as already described, except a current code will clear itself after 40 driving cycles if you have fixed the problem. A pending code clears itself immediately on the next drive cycle if the issue isn't detected again. Pending and current codes can also be cleared by Techstream, or by disconnecting the battery.

    You can't clear a permanent code at all, except by fixing the problem. For each code, there is a confirmation driving pattern that lets the ECM judge whether the problem is fixed, and a permanent code goes away after 3 consecutive drive cycles produce a normal judgment. Nothing else clears it.

    Also, the check-engine light will go out after 3 consecutive normal judgments (even though the current code sticks around for 40, or until you clear it).

    Any time you do the Techstream clear, or the battery disconnect, you also lose all stored freeze-frame data and reset all the emissions monitors to incomplete.
    pumpanddump and douglasjre like this.