TPS Code Won't Go Away

So you've tested the throttle position sensor (TPS) and according to the test results, the TPS is good... But the CHECK ENGINE LIGHT keeps coming back on even after you erased the Diagnostic Trouble Code (DTC) from the computer's (PCM) memory. Well, here are a couple of suggestions that might inspire your next diagnostic move:

  1. 1

    The throttle plate's idle-stop screw's factory adjustment has been altered so that the engine could be idled up and mask a miss/misfire and/or rough idle. This increases the TP sensor's signal to the PCM. The PCM doesn't like it and light ups the CHECK ENGINE LIGHT (CEL).

  2. 2

    The throttle cable is binding and causing the throttle plate to not fully close.

    1. This can be verified by simply having someone inside the vehicle pushing the accelerator cable to the floor and releasing it, with the engine OFF, while you visually check that the throttle plate and cable are not getting stuck somewhere in their travel.
  3. 3

    The TPS is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:

    1. I have found that the best way to test these intermittents is to road-test the vehicle with the multimeter hooked up to the TP signal Wire with a long wire so that I can comfortably observe the signal going up and down as I or someone else drives.
  4. 4

    The TP sensor's connector is BAD, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.

Symptoms Of A BAD TPS

You'll have the CHECK ENGINE LIGHT on, for sure, on your instrument cluster and one of several of the following symptoms:

  1. 1

    TPS Diagnostic Trouble Codes (DTCs) stored in the vehicle's computer's memory.

  2. 2

    Really BAD gas mileage. You know that it's not the price of Gasoline that has you thinking that your 3.8L V6 equipped GM vehicle is costing you more at the pump.

  3. 3

    Transmission does not shift out of second gear. Now, this doesn't happen very often, but it happens.

  4. 4

    No power and/or hesitation as you accelerate the vehicle. It feels like all of a sudden someone cut the power out momentarily as you step on the gas to get the vehicle moving.

How The Throttle Position Sensor Works

The throttle position sensor's job is to measure the angle of the throttle. So here, in a nutshell, is how the throttle position sensor works when you crank and start your 3.8L GM V6 equipped vehicle:

  1. 1

    The fuel injection computer supplies 5 Volts and ground to the throttle position sensor.

  2. 2

    Now, since the throttle is closed, the TPS (with power and ground supplied) sends the PCM (Powertrain Control Module=Fuel Injection Computer) a DC voltage signal of about .5 to .8 Volt. This value is what the PCM associates with a closed throttle.

  1. 3

    Once you throw the car in drive and accelerate the car, the throttle opens and the throttle position sensor immediately sends this change of the throttle angle as an INCREASING voltage signal to the PCM.

  2. 4

    With this increasing voltage signal, the PCM knows its time to inject more fuel, advance ignition timing, and a host of other things it has to do to keep your 3.8L GM equipped vehicle running optimally.

  3. 5

    As you let go off the accelerator pedal to slow down, the throttle plate closes and of course the TP sensor sends the info to the PCM as it returns to its base voltage signal, till the whole cycle begins again.

Pretty easy stuff? The cool thing is that the tests to check out the TP sensor's performance are as easy too. Now, since you'll be working in the engine compartment take all necessary safety precautions and use common sense. OK, enough of my yakking, let's get this show on the road... go to TEST 1.