Question about Pontiac Grand Am

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Po121 code repl tps 3 times. 1.25 v on low and 5v on high. when i clear codes rpm idle goes 600 to 1000. help

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  • bornagain1 May 01, 2010

    ive disc. battery for .5 hour and no difference also clear codes comes rite back you say maf sensor?

  • bornagain1 May 01, 2010

    how do i chk ground

  • bornagain1 May 01, 2010

    how do u test ground on tps

  • bornagain1 May 01, 2010

    98 grand am 3.1 110k checked on harness near tps. what is pcm? also how do you check fuel rims with scantool? how do you ck ground at tps

  • bornagain1 May 02, 2010

    is there a way to test computer or do i just buy new one and hope

  • bornagain1 May 02, 2010

    now have to clean throttle body system was going to go and buy new computer but thanx wont now didnt know also 2 hrs on comp clear good to know will talk next weekend on how goes. God bless you my friend

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2 Answers

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  • Pontiac Master
  • 4,369 Answers

Sometimes the code is generated because of a base engine problem & the sensor is just responding. Check the ground for th ecircuit and clean the idle air passages in the Idle Air Control.
Then disconnect batery to clear computer memory so it can learn all over again. Also clean MAF (Mass Air Flow Sensor) with carb cleaner.

Posted on May 01, 2010

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  • gerry bissi May 02, 2010

    The MAF, Mass Air Flow sensor is between the ar cleaner and throttle body on the air intake hose. It usualy has a couple of screws to remove it. Spray the 2 short wires inside with carb cleaner or MAF cleaner several times. Coated wires cause a lean mixture.



    The TPS has a contact that slides along a resister and sends a voltage reading between 1 to 5 volts to the PCM (Powertrain Control Module). If the TPS has inermittent open circuits along the resister, it confuses the PCM (also called the engine computer).



    Ask if your TPS has to be adjusted when replaced, most do. There will be a closed throttle ohm or voltage reading and one for wide open throttle too.



    If you have replaced it 3 times, I think it could be a base engine issue like plugged passages in the idle air control on the throttle body. Also, a slight chance you have a bad PCV valve as they allow a metered amount of air to flow into the intake manifold at ilde too. They are about $5.



    Do not replace the PCM, they are never bad. 90% of those replaced are tested and found not defective and resold again.



    Don't worry about the ground if the closed & wide open throttle TPS readings are correct.



    TPS also affects sfit quailty too.

  • gerry bissi May 02, 2010

    some expensive scan tools will give you a fuel pressure reading, around 40 psi is normal, but most don't. Need a factory scan tool for that.

  • gerry bissi May 02, 2010

    Forgot to mention, disconnecting the battery for 2 hours will clear the "Keep Alive Memory" so the computer can re-learn the new parts and set the fuel mixture correctly.

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  • Master
  • 661 Answers

Is the scantool reading those voltages at the PCM? Also what year vehicle is this?
Usually this means that either the TPS is bad or the PCM is bad.
Using the scantool you should check the fuel trims.
Also check for vacuum leaks and doublecheck the other sensors are reading properly. Like temp, barometric pressure. etc.

Posted on May 01, 2010

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  • Bryan Fullerton
    Bryan Fullerton May 01, 2010

    ok a 98 is and odb2 computer. The PCM is the powertrain control management. Or you can call it an ECM. Most people just call it the "computer" since it runs your engine and transmission.
    Scantools are all different so I cant tell you how to operate yours. However odb2 cars will output the fuel trims. For instance if the O2 sensor detects a lean condition the computer will take that info and give it a positive trim to the fuel. An example for that would be a vacuum leak which would show as +fuel trim. Personally I use a laptop to interface with computers. Even my '93 suburban shows fuel trim.

    Really high or low trims are indicative of a fuel or ignition problem. If a sensor goes bad it can feed false information to the computer. The computer has to decide if the information is bad or good and sometimes it gets it right and sometimes it doesn't. If your scantool can show you tps numbers while the engine is running then there isnt a problem with the TPS. However if the computer is getting fluctuating vacuum readings or MAF readings then it may think the TPS is lying. Or perhaps the speed sensors is giving bad readings that dont match with the TPS. Either way the computer can flag the TPS as being the liar. It would then throw a TPS bad code.


  • Bryan Fullerton
    Bryan Fullerton May 02, 2010

    oh you got me on that one. I'd think it would take a special tool to test that. Unless of course you were really into the nitty gritty of dissecting them.
    First try your local parts house and see if they know of a way to test them.
    Second if you can get your hands on a good used one you could test it that way.

    I usually end up getting a good used one if I have a vehicle that I have doubts about.
    Well I used to do that more often before I got a sensor tester. Now I usually go test all the different sensors first with the sensor tester.


  • Bryan Fullerton
    Bryan Fullerton May 02, 2010

    I'd like to know how it goes. Hope it all works out.



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