1999 Pontiac Grand Prix OBD2 codes

  • 3 POSTS
  • 3.8L
  • V8
  • 2WD

The diagnostic tool code reads PO122 which is throttle/pedal position sensor A circuit low input.
What exactly does that mean? Could you maybe explain it in easier terms or what I could possibly do to or how I would go about getting the problem fixed..

The only other code it put out was PO650 which is the malfunction indicator light control circuit.
Again any suggestions on how to fix that also.

Would greatly appreciate any feedback as I am trying to fix this on my own if I can due to being single and reallly no funds to pay someone else or to take it to a garage to have them look at it.

Id like to thank you for taking your time to read my questions or posting.

Look forward to hearing back from you.


Do you
have the same problem?
Thursday, June 26th, 2014 AT 8:59 PM

1 Reply

  • 29,768 POSTS

The throttle position sensor is just a big resistor with a movable contact that can select any spot on it. One end is fed with 5.0 volts, and the other end goes to ground through the Engine Computer. Ground is, in effect, 0 volts but due to the monitoring circuitry, you'll actually find 0.2 volts on it. That's way more than you need to know for this problem.

The signal wire is connected to the movable contact, but there are mechanical stops that limit its travel to between 0.5 volts and 4.5 volts. That means that at idle you'll find 0.5 volts on the signal wire, and as you open the throttle, the voltage reading will go higher until you reach 4.5 volts at wide-open-throttle. The Engine Computer will not set a diagnostic fault code for that sensor as long as the signal voltage stays between 0.5 and 4.5 volts.

One way to get a low voltage fault code is to have a break in the 5.0 volt supply. That can be a break inside the sensor, but more commonly it will be due to a stretched or corroded terminal in a connector, or a break in a wire.

A second way to get that code is to have the signal wire shorted to ground, meaning it has rubbed through and the wire is touching the metal engine or body, ... Or that wire could have a break in it like in the last paragraph, but that depends on the car model. With a break in that signal wire, you don't know what the voltage will "float" to inside the computer since it's inter-connected to all the other circuitry, so to prevent the computer from acting on some erroneous voltage that is within the 0.5 to 4.5 volt range, they use a "pull-up" or "pull-down" resistor to force the voltage to go to a bad value the computer will recognize and set the appropriate code. Most cars use a pull-up resistor. They are so big electrically that they have no affect on the circuit when it's working properly. With a break in the signal wire, however, it will put 5.0 volts on it and the computer will set a fault code for "TPS voltage high".

Regardless, start by checking the wires for that sensor. If you erase the fault code and it comes right back, it's almost always a wiring problem or the sensor was disconnected while the ignition switch was on. If the code doesn't set again for a while, it's more likely there's a loose contact between mating terminals in a connector or there could be a speck of dirt getting caught under the movable contact in the sensor. The sensor itself will be the cause of the code about half of the time. The other half it's wiring issues.

Understand too that the 0.5 volts and 4.5 volts in my story is the theory part of it. In actual practice you might find 0.43 volts and 4.24 volts. The point is the signal voltage must never go all the way to 0.0 or 5.0 volts.

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Thursday, June 26th, 2014 AT 9:29 PM

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