Code P0122

Hello, this code for the TPS (low circuit input). This may be a wiring issue with this code. You'll have to check the circuits to the TPS. The grey/black wire is the 5 volt reference wire. The black wire is the ground and the dark blue wire is the signal wire going back to the ECM. With the key on, not running. You can back probe the wire connector with a T pin and check the voltage. If you back probe the wire, put the voltmeter's black lead on the battery negative and the red lead on the wire you're testing. The black wire (ground) should have 0 volts. With the key on and back probed on the Signal Wire (dark blue wire) you can press the gas pedal down slowly and watch the voltage raise on the multimeter, it will be a low voltage near 0-0.5volts and raise to near 4.5-5volts. Try these tests and let us know what results you get. Also take off the air boot and check the throttle plates, make sure they are fully closed with the car off. Carbon build up on the throttle plates can cause them to stick open. But this code is pointing to a voltage issue the ECM is seeing and that's why its setting the code. I have added the full Diagnostic Procedure as well.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/engine-idles-too-high

Okay, that's a good first step. Does the grey/black wire have 5 volts with the other multimeter lead going to battery ground? With such a low voltage reading on the signal wire, i'm willing to bet your missing the 5 volt reference on the grey/black wire. Make sure you test with the key in the On position. If your missing the 5 volts, then we will need to chase down that issue. One other question, does the check engine light come on when you turn the key to "on", and then go off? It should come on for about 2 seconds then go off. That will tell you if the ECM (engine computer is being powered up). The last test is to check the grey/black wire with the TPS unplugged, so you'll be testing the harness side coming from the ECM.

The grey/black wire reads 11.99volts with the other lead on battery negative? This is the same wire that reads 5 volts at the TPS sensor?

A side note, when checking a ground wire, (the black wire), you want to leave the meter set on voltage, with the key still in "on" position, you'll then want to back probe the black wire and other meter lead going to battery negative. This is called a voltage drop test. So you will be testing how much voltage is lost from the sensor to the battery negative. It should read almost nothing, 0.050 volts or less. Doing a resistance test will not show a problem on a wire, that might only occur when the circuit is "on" and loaded.

I'm not sure if you're on the right wire. Because if you're reading 5 volts on grey/black wire at the sensor to battery negative, You would be reading the same 11.99 volts at the sensor as well. If you get what i'm saying. Unless you're taking a reading across the connector, one lead on one side and the other lead on the opposite side. Then I would pull that connector apart and see if its corroded inside.

Yeah, as the wires age too, the colors can get hard to distinguish between each other, but we're still on the right path here. The signal wire being out of range. I would go back and recheck the ground wire, if the sensor has the 5 volt reference, and has a good ground, then its probably another bad sensor. It happens more than you would think, but voltage drop that ground wire with the key on, so red lead on the black wire and other lead on battery negative. This is with the sensor plugged in. Try this test with just key on and running test and see what you get. It should be very close to 0 volts. The fact that you're not getting any reading while pushing the pedal down, that's our issue here. Does this car have a throttle cable going to the Throttle Body? Or is it just wires going to it? If it has no cable, then it is a drive by wire system. Meaning you may have a bad throttle body. If the car does not have a throttle cable, then take the air boot off the throttle body and depress the gas pedal, you should be able to see the throttle plates move with the key on. See what you get with testing the Ground (black wire) and watch the throttle plates move. See what you get there.

Okay, that's strange that it started dropping off like that. How far down did it go? And with your foot off the pedal, really those throttle plates should be fully closed. Did you get to voltage drop the Ground wire at all? So you are getting a reading on the Signal wire now though it sounds like. It shouldn't be dropping off though. Does this high idle happen all the time? I'm just trying to narrow down the possibilities here. Either the throttle plates aren't closing all the way allowing extra air in, you have a vacuum leak, possibly a bad Idle Air Control Valve,

try unplugging the Mass Air Flow Sensor on the beginning of the Air Boot, take the air boot off, start the car, if it idles high, put your finger over the hole just in front of the throttle plates. See if the idle comes down. That's the hole for Idle Air Valve. We're a bit off track here because of the TPS voltage readings your getting. You can try replacing the Idle Air Control Valve, they're about $35.00 at AutoZone. They have a high failure rate. But i'm not %100 on that because I'm not really sure what voltage readings you're getting at the TPS.

Okay good, at least your getting some solid readings now, I cant explain the voltage drop off like and hopefully its not an ECM problem, But for now we'll keep checking other things.
The hole for the Idle Air Control Valve (IAC) is right in front of the throttle plates, so it will be the neck of the throttle body.
So technically when you unplug the Mass Air Flow sensor, the ECM should go into a default fuel strategy and the engine should run okay, (This is on most cars, but i'm not 100% on Chevys) But I wanted to see if the idle would come down when you put your finger over the IAC hole. This hole just bypasses the throttle plates and allows the ECM to control the idle by moving a Stepper motor back and forth inside the IAC. If its stuck open, the idle should come down by doing this. If the car just stalls out, we're probably going to check for a vacuum leak. Do me a favor and take a picture of the engine layout and post it here, so I can direct you a little easier.
I am concerned with that voltage drop off though, reason being, is this Code p0122 is for a Low Input signal from the TPS to the ECM. I'm wondering about a possible bad ECM ground here as well.
I know it's a lot to take in at once, But I think while your messing around with the car, you may want to keep your multimeter set up on that Signal wire (dark blue wire). If you can with an alligator clip to hold your lead on the pin. These are one of my pair I use. Since you're going to be a pro at testing wires when we're done here.
So does the car surge when your driving? For instance when you have the throttle plates open with your foot on the gas? Or does it go away? Reason i'm asking is a vacuum leak will go away when you open up the throttle plates and more air is moving in. Vacuum leaks will be more noticeable at idle.
So try your finger over that IAC hole, take me a picture of the engine, and monitor that Signal wire and see if the voltage drops off like that again. If it does we're going to have to start checking the ECM power and ground wires.

I should have added this, but having the Mass Air Flow sensor unplugged is going to set another code, so just ignore that when you have this rescanned for new codes. Have them just erase the codes, if you have any parts replaced.
I'm going to throw this out there right now, Here are the Grounds for PCM/ECM. They're right near the starter motor in front of the engine, so the front of the engine means where the main drive belts are. But check these Grounds, I saw this picture and they're all bulked together, probably on one stud with a nut on it. If there's any rust or corrosion, take the nut or bolt off and clean the heck out of this Ground location. I remember a car like this having an issue with the ECM and it was these grounds that were the problem.
They put all the PCM grounds together like this, and if one is bad it effects the whole lot of them. Black/white wires.

Sorry for all the edits here, Just to see if this works, if you do have a new TPS sensor on there, try unhooking the battery negative for a few minutes, the hook it back up, This will erase the ECM memory, lets see if that works. This is along the lines of erasing a code after a part is replaced,
I'm adding an installation tip I just found, I know it's for the 3.8L, but the 3.1L is very similar.

Okay, thanks for the pictures. Well we've found the real issue at least. Its strange that you're even getting any reading on the signal wire at all. I think you're going to have to check that 5 volt reference wire while this problem is occurring. My head is telling me you may have another sensor that is shorted out and pulling the 5 volts low or a corroded connection somewhere. The ECM needs a full 12volts to operate. I'm not sure its a bad ground.
The reason for this is, if you had a bad ground wire, the signal would be high because its not being pulled to ground, but since its a low signal, either that 5 volts is low, or power to the ECM/PCM is low. This is definitely not an average problem. I'm glad you've been able to do these tests so far. This is getting to be some tough diagnostics.

So the next step here is going to be checking the 5 volt wire at the sensor when you're not getting a reaction out of the TPS. I don't know if this is a problem that is always happening or if it is intermittent. Since you did have a reading at one point during all this.

If you start the car, does the throttle respond while its running? Even if it has a high idle?

I would even check that your battery has a good charge, because at this point with all the testing with just the key on, the battery is going to be low on charge. So start the car and let the battery charge up before anymore testing. Make sure at the alternator post to battery negative its charging at least 13.5-14volts.
I will be really embarrassed if this turns out to be a charging issue.

So next steps, charge up the battery, check that 5volts while the problem is happening, And is this the Base Malibu model or the LS model?

I'm giving you the power diagrams to check the ECM. So what you'll have to do on these, since there might be more some wires that are the same color, is just make sure the wire you checking has the same color wires next to it as on the diagram.

So the 1st Diagram is the battery voltage wire is an "Orange" wire next to it a Pink wire, and a Purple wire on the other side. The Orange wire should be battery voltage all the time (12v), And the Purple wire is Battery Voltage while the key is in the On Position (12v).
These are all highlighted in yellow.

The grey wire above those, you'll see, is a 5 Volt reference wire. Check all these after the battery is good and charged up.

The 2nd diagram is all 5 volt tests, Key On is fine for these.

The 3rd diagram is one 5volt ref, and two 12v ref.
I'm assuming you know where the ECM/PCM is because you said you traced a wire back to it, which is great by the way.

Just really make sure that battery is good, its really important that its fully charged for anymore testing. Let me know what you find.

And one thing, these tests we're doing are not loaded circuit tests. Meaning we're not stressing the circuits right now, which is what I would be doing in a shop. But for right now, just do these basic checks and we'll keep going from there. Doing these tests while the car is running would be even better, but if you're worried about the idle being all over the place I understand.
If everything checks out, check the wires while doing some wiggle tests on the harness, sometimes while moving the harness around a little, a problem will reveal itself.

In your pictures, the 3rd one you asked about, looks like an ignition module for the coils. You have spark, so go right to the ECM now. Its labeled as just below the left side of the dashboard, so driver side. Its going to have a ton or wires running to it like in the diagrams. I know there is 3 of them, but there just broken up that way. All the pictures run to the same ECM.
I really hope you didn't just get a bad part from the auto store. Its been happening a lot more lately. Just junk brand new parts.

Just wanted to throw more to think about, sorry for this, but while tracing wires back from the TPS to the ECM, check as much of the wiring as you possibly can. I know you've got an odd situation here. But any contact points where the harness touches any part of the engine or frame, where it can rub threw after years of vibration and movement.

Hey, I was going through your comments here, if you put the old TPS sensor back on the vehicle, I really think you should try a new sensor. We get brand new sensors that are bad all the time. Try another TPS and have the codes cleared or disconnect the negative battery cable for a few minutes. You've tested enough that we should have come across a problem by now. Bring the new TPS back that you bought and tell them its bad and get another and try it.
The memory needs to be erased with a new sensor in there. If you still have problems at least we can absolutely rule out the TPS.