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  1. #1
    Light Throttle bjamick's Avatar
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    PO128, thermostat code. New therm and still throws code?

    Well this happened soon after I bought the car. The temp guage would just fall to 0 and maybe a day later it would throw a code. So i would check it with my HPtuners and delete the code and the temp guage worked for maybe another few weeks. Then it would just do it again.

    Well this would cause the fans to stay on constantly, so I figured I would try and fix it. So I bought a Hypertech 160* thermostat and installed it and everything worked how its supposed to. Well last night on my way home the temp guage fell to 0 again. So I went ahead and deleted the code again (PO128) and everything worked.

    Anybody got an idea what could be going on? I just figured it was a bad thermostat but the one thats in it is brand new. Is there a sensor somewhere that could've just went bad? Its a 2002 3.8 firebird if it matters.

    Any help would be greatly appreciated.
    Thanks

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  3. #2
    Full Throttle chadm_88's Avatar
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    A bad thermostat wouldn't make your temp read zero, it isn't connected to your gauge. It is probably a bad ECT (engine coolant temp) sensor.

    pn SU109 - 11.99 at autozone
    Last edited by chadm_88; 09-22-2008 at 11:25 AM.

  4. #3
    Moderator 13secv6's Avatar
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    The car will do that with a stuck open thermostat or the 160 thermostat... its not that its not working... its that it is working too good. The code is about the engine not heating up fast enough.... the 160 just keeps the car too cool for the PCM to like... some cars can get away with it, some cannot... Mine is the same as yours.... summers it fine but on cool nights it will drop the gauges and kick the fans after driving around for a while in cold air

    Drew--F-Body General Tech Moderator



  5. #4
    Light Throttle bjamick's Avatar
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    Thats weird, well it was a cold night. But when I went to replace it, whoeve owned it before me didn't even have a thermostat in it.

  6. #5
    I'm goin' nonlinear! Jon's Avatar
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    Put a stock temp T-Stat in there and tell us what doesn't come on (aka your code).
    2007 Honda Civic EX Coupe | 2005 Pontiac Grand Prix GT
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  7. #6
    Full Throttle chadm_88's Avatar
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    Does the temp gauge go up any before it drops? Because a bad (or nonexistent) thermostat wouldn't just randomly/suddenly make the temp read zero. It would just take forever to warm up. There is a problem in the ECT sensor circuit, whether it be the sensor it self, or any wiring going to the PCM.

    I do agree with Jon though, that you need a stock temp thermostat.

  8. #7
    Light Throttle bjamick's Avatar
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    It works fine until it gets ready to throw the code.

  9. #8
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    Quote Originally Posted by chadm_88 View Post
    Does the temp gauge go up any before it drops? Because a bad (or nonexistent) thermostat wouldn't just randomly/suddenly make the temp read zero. It would just take forever to warm up. There is a problem in the ECT sensor circuit, whether it be the sensor it self, or any wiring going to the PCM.
    Yes it will. A thermostat stuck open, in cases will make your temp gauge stay at zero. Happened to me.
    Jay

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  10. #9
    Full Throttle chadm_88's Avatar
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    No I mean, the way I take it when he describes it, the temp gauge goes up, then suddenly drops to zero. Yeah, having a stuck open thermostat will make the temp stay low, I know that lol. It just wouldn't go up then suddenly drop down. Maybe I'm misunderstanding the way hes describing it.

  11. #10
    Replace your 160* with a 180* as stated above and clear the code. Be willing to bet the code won't come back. I forgot what it was, but something on the 01+ didn't like 160*.....

  12. #11
    I'm goin' nonlinear! Jon's Avatar
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    I wouldn't even suggest a 180* T-Stat. Get a stock 195* and go from there.
    2007 Honda Civic EX Coupe | 2005 Pontiac Grand Prix GT
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  13. #12
    Light Throttle bjamick's Avatar
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    Hmm that is really weird. Well the thing is the car didn't even have a thermostat in it. I found that out when I went to put in the 160*.

    Let me explain it again for those of you that dont get what i'm saying.

    Lets say I clear the code one morning. The temp guage and fans will work just as any other cars should. Everything is fine. So fast forward about 2 weeks or something like that. Lets say i'm drving down the interstate, everything still working fine, temp guage shows under 210. Then all of a sudden it will just drop to the bottom of the guage. It wont trip the SES light for a day or 2. Once the temp guage drops to 0 or at the bottom of the guage the fans automatically turn on and stay on the whole time your driving.

    Then it just starts over from the beginning again . The car still runs fine either way.

  14. #13
    Full Throttle chadm_88's Avatar
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    Yeah, that's what I thought was happening. Running without a thermostat or a lower than recommended one will make your car take longer to warm up and may run cooler than normal all the time, but there is no way that would make it just drop to zero after it has warmed up. Its not electrically or mechanically connected to your gauge at all. However, having a bad or dropping signal from your ECT sensor WOULD make it drop suddenly.

    I would recommend testing the sensor with an ohm meter to make sure its bad. Otherwise, there's another problem wrong with the ECT circuit, maybe a loose ground, corroded wire, etc. And you still need a 195* thermostat to insure that you are running your car at the right temp. It comes with that for a reason, and that's because it runs best at that temp.

  15. #14
    Moderator 13secv6's Avatar
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    ITS BECAUSE HE HAS A 160* THERMOSTAT!! not a gauge issue, or sensor issue.... yes, its not electronically controlled, BUT the PCM relys on Coolant temps for certain values of spark, fuel etc.... if its not high enough, or is taking too long to heat up the car freaks out and the zeroed gauge and fans are the result....

    This is a known problem with 00+ cars.... just get a 180* or higer in there and you wont have any issues....

    Drew--F-Body General Tech Moderator



  16. #15
    I'm goin' nonlinear! Jon's Avatar
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    He's been told what to do.

    Some people just can't be helped.
    2007 Honda Civic EX Coupe | 2005 Pontiac Grand Prix GT
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