read and reset check engine light

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by prius4ed, Feb 19, 2011.

  1. prius4ed

    prius4ed Member

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    My CEL came on recently. I've read posts that to reset it, one would disconnect the battery neg cable for 5 mins and reconnect it. [1] What about disconnecting the positive battery cable instead? I'm only asking b/c it's easier to reach and I've done it a few times but my airbag light is still on. Wondering if the neg battery cable is more effective electronically speaking. I recently wanted to check a flat front left tire for a nail, so I raised that tire with a floor jack and put the car in "N" (power on) and disconnected the battery + cable (thinking it'd leave the car in "N" with power off). To my surprise, the dash lights were still lit, as if the cable was still connected! I went ahead and rotated the tire very slowly checking for nails. I hope I didn't throw a code or damage anything by doing this. I should have just removed the tire from the car, but I was too lazy and this is how we did it in the old days with dumber cars.
    [2] Before resetting CEL, can I read the code without a scanner (is there a way to make the CEL flash the code by jumping something on the DLC)? My airbag light came on a few weeks before the CEL, but that's probably unrelated.
     
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    1. If you look at the negative battery cable where it bolts to the body, you will find this is easy to unbolt and a quick way to disconnect the 12V battery. The reason why it is not suggested to remove the positive battery cable is because if you short the positive terminal to ground with your wrench (while the negative side is still connected) you will cause a very big spark which is a safety hazard.

    Once the Prius is READY, you can disconnect the 12V battery with no obvious impact to the car's driveability. This is because the DC/DC converter is maintaining the 12V bus at ~13.8VDC. Obviously, once the car is made IG-OFF without the battery connected, then it is not possible to power up again while the battery remains disconnected.

    It is not possible to leave the Prius in N with the power off without removing power to the ECU that powers the parking pawl. If you care to do that then you'll need to do homework by studying the relevant wiring diagram.

    However, if you raise the entire front end you can rotate the front tires when the car is IG-OFF and in P. One tire will rotate in one direction while the other tire will rotate in the opposite direction.

    2. If the check engine light is on, you cannot read the engine ECU DTC without using an OBD-II code reader.
     
  3. pEEf

    pEEf Engineer - EV nut

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    There is an easy and safe way to disconnect the battery without tools. Unsnap the red plastic cover (it slides forward once the 3 tabs are released) and disconnect the black cable with the white connector. It is the large one that plugs into the black battery terminal block housing. There is a release tab located under it, and when pressed it will allow the connector to easily unplug:

    [​IMG]

    [​IMG]


    Once disconnected (and the car is off!), the ECU's will all reset after about 30 seconds, then you can re-connect the plug. The ECM (Engine Control Module) which controls the Engine Light will clear, but the SRS light will not. (this requires a proper scantool)

    Note that you must read any powertrain codes before unplugging the battery, otherwise they will be lost.
     
    Greg Grieve, dave77 and uart like this.
  4. violinguy

    violinguy Junior Member

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    Just a quick not on the Airbag light, my wife has a penchant for storing small articles under both front seats of her 2005. Periodically I will see her airbag light on and (from one of these posts) it is an easy fix where just jiggling the connections under the seat (with the car off) 100% of the time has fixed the light and this has happened 5-10X!!! Seems to be a over-sensitive area electronically, but at least it is simple to fix! Because of the sensitivity, I recommend not putting anything under the seats!
     
  5. stockdaddy

    stockdaddy Member

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    My airbag light is on also, i'll try that fix.

    Ya that is the easiest way to dc the battery without tools.
     
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