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I am having issues with unsteady RPMs that feel as if the car is "2-stepping" at about 1500-2000 RPMs and I am getting codes P1120, P1220 - Throttle Position Sensor 1 & 2 Circuit. Over the course of 100-150 miles of driving the problem worsened. The RPMs typically became unsteady at around 3000-4000 RPMs, and over the course of 100-150 miles of driving, the car is "2-stepping" around 1500-2000 RPMs. I have a ls2 throttle body on my ls1 corvette along with the ls1 to ls2 throttle body harness adapter. From my understanding, the cause of the problem or multiple problems can be:
APP (accelerator pedal position) sensor
TPS (throttle position sensor)
TAC (throttle actuator control) module
Faulty wiring/connection
From my understanding, throttle position sensors 1 & 2 are both integrated into the construction of the throttle body. If this is the problem, I need to replace the ls2 throttle body for $500+ or perform "electrical surgery" on the ls2 throttle body. If the APP sensor is the problem, then I need to replace the pedal assembly for a fraction of the price of a ls2 throttle body (hopefully this is the problem). If the TAC module is the problem, then I need to replace the TAC. I'm not too sure which of these 3 items is problematic (or faulty wiring/connections).
I have attached a HPL file from my VCM scanner that shows my throttle position percentage and accelerator pedal position percentage. This test was with the ignition off and accessories on. When the accelerator pedal is fully depressed, my throttle position is at 100% and my accelerator pedal position is at 112%. How can my APP be above 100%? Is that normal? In the test file, I slowly depress the pedal, and if you pay attention to the throttle position percentage, as it reaches 20-30%, the throttle seems to not move linearly with the accelerator pedal position.
My car is tuned with an LSA blower and I'm not sure if my tuner changed parameters with the pedal position (see attached HPT file) or if changing APP parameters are even possible in HPT.
From this information, what is the problem? The TPS? The APP sensor? Or maybe the TAC?
How should I go about diagnosing the problem?
@Bill Curlee@Speedy007@whatyacallit@C5 Diag I hope you're all still active around these forums because you all gave some insightful information regarding this issue I had nearly 4 years ago (this is the same problem from a pervious thread 4 years ago but I narrowed the problem down).
Best,
bbuddine
Last edited by bbuddine; Aug 12, 2024 at 09:33 PM.
Seems like the car may be in "limp home mode" because of the TPS throwing those codes. It's there to protect you and the car. I would like you to add these parameters in your VCM scanner to both your "Channels column" and your "Chart vs. time" graphs.
1. Throttle position sensor [Voltage]
2. Throttle position sensor 2 [Voltage]
Try and clear the codes. Do the same test you did and lets see what the PCM is seeing.
Also on a side note it in VCM editor it looks like you're still running a 1 bar MAP sensor. How many LBS of boost are you running?
Also referring to your question about your APP. In VCM editor if you go under Engine/Airflow/Electronic throttle/Desired throttle area you will see why your throttle position is different than your pedal position. Examples: At 25% pedal position your map shows only 3.79% at 50% pedal position it's at only 15.06% at 75% it's at 46.78% Not until you reach 94% pedal position wil you get 100% throttle opening. All this to say that there is nothing wrong with your APP so cross that off the list.
Don't mess with these settings unless you REALLY know what you're doing!
[ECM] 33000 - Desired Pedal Area - Normal: Desired throttle area based on pedal position and barometric pressure (pedal to throttle map). WARNING!!! Modify this table at OWN RISK. Incompatibility of this calibration with the vehicle TAC module calibration may render PCM permanently inoperable.
Seems like the car may be in "limp home mode" because of the TPS throwing those codes. It's there to protect you and the car. I would like you to add these parameters in your VCM scanner to both your "Channels column" and your "Chart vs. time" graphs.
1. Throttle position sensor [Voltage]
2. Throttle position sensor 2 [Voltage]
Try and clear the codes. Do the same test you did and lets see what the PCM is seeing.
Also on a side note it in VCM editor it looks like you're still running a 1 bar MAP sensor. How many LBS of boost are you running?
Also referring to your question about your APP. In VCM editor if you go under Engine/Airflow/Electronic throttle/Desired throttle area you will see why your throttle position is different than your pedal position. Examples: At 25% pedal position your map shows only 3.79% at 50% pedal position it's at only 15.06% at 75% it's at 46.78% Not until you reach 94% pedal position wil you get 100% throttle opening. All this to say that there is nothing wrong with your APP so cross that off the list.
Don't mess with these settings unless you REALLY know what you're doing!
[ECM] 33000 - Desired Pedal Area - Normal: Desired throttle area based on pedal position and barometric pressure (pedal to throttle map). WARNING!!! Modify this table at OWN RISK. Incompatibility of this calibration with the vehicle TAC module calibration may render PCM permanently inoperable.
Good luck
See attached HPL file. There was not an existing channel for a throttle position sensor 2 that I could find in the drop down menu (see below image) but there was a option to add throttle position sensor 2 in the chart vs time graph. Throttle position sensor 1 was reading zero volts across the span of 0%-100% of the accelerator pedal and looking back at the test file, it looks like the file didn't save the chart vs time graph for TPS voltage because it was at zero volts? Maybe I I didn't log something correctly? After I connected the vehicle to the scanner , I relogged the VCM suite to refresh any parameters that may not have been active. I am running approximately 11 lbs of boost. I have a 3 bar map sensor, but I just haven't tuned the car to a 3 bar open loop because I don't know how to do that and I want to stay away from bricking my ECM.
Last edited by bbuddine; Mar 25, 2023 at 03:09 AM.
I just tried running the test on my 2004 vert to validate. It seems HP Tuners didn't add many of the channels for the TPS to the "Channels list" but if you right click on the "CHart vs time" and choose "charts layout" then pick any channel then change the parameter the "parameter selector" pops up. If you type in TPS you'll see the list of voltages for the tps I was referring to. Anyway, I know this does you no good. Someone could put in a ticket with HP to have them added to the channels list I suppose.
One piece of info for you.....You wrote (Throttle position sensor 1 was reading zero volts across the span of 0%-100% of the accelerator pedal and looking back at the test file, it looks like the file didn't save the chart vs time graph for TPS voltage because it was at zero volts? Maybe I I didn't log something correctly?)
When I was testing I noticed that mine showed 0 volts too. My car runs fine so It's probably a problem with HP Tuners.
I see a lot of issues on here with these LS2 throttle bodies. The code setting criteria for both DTC's is that the TPS 1 and TPS2 voltages are LESS than 0.13 volts or MORE than 4.87 volts for less than 1 second or a voltage spike exceeding those voltages and it's looking for voltages and not percentages...even it it drops or increases momentarily a voltmeter is too slow to see these minute voltage changes...you need a scope (see waveform below). The first thing I would check is the 5 volt reference on both sensors...disconnect the harness and the following colors are for the OE harness side with key ON...TPS1 is a dark green wire and with your DVOM use the purple wire (low ref) as your DVOM ground...you should see 5 volts. TPS2 is the yellow/black and the low ref (ground) is the white wire...again you should see 5 volts. Connect the harness back up and backprobe at the OE side of the harness adapter because these voltages is what the computer is seeing from your LS2 TB...with key ON at idle (foot off the pedal) TPS1 voltage should be 0.13 to 0.67 volts on the DARK BLUE (signal wire) and on TPS2 you should see 4.3 to 4.8 volts on the PINK signal wire...now with the pedal down to the floor you should see 4.09 to 4.87 volts on the DARK BLUE TPS1 signal and 0.13 to 1.0 volts on the TPS2 PINK signal wire...these TPS sensors are just 2 potentiometers that work in opposite directions....TPS1 goes low to high voltage and TPS2 goes high to low as the throttle is depressed.
I went ahead and purchased a LS2 throttle body. I'm assuming that this is the issue. If it's not the issue, I can return it and I will report back. Thanks for the advice though.
I just tried running the test on my 2004 vert to validate. It seems HP Tuners didn't add many of the channels for the TPS to the "Channels list" but if you right click on the "CHart vs time" and choose "charts layout" then pick any channel then change the parameter the "parameter selector" pops up. If you type in TPS you'll see the list of voltages for the tps I was referring to. Anyway, I know this does you no good. Someone could put in a ticket with HP to have them added to the channels list I suppose.
One piece of info for you.....You wrote (Throttle position sensor 1 was reading zero volts across the span of 0%-100% of the accelerator pedal and looking back at the test file, it looks like the file didn't save the chart vs time graph for TPS voltage because it was at zero volts? Maybe I I didn't log something correctly?)
When I was testing I noticed that mine showed 0 volts too. My car runs fine so It's probably a problem with HP Tuners.
Thanks for testing on your own to validate the problem with the VCM scanner.
You should have taken C5 Diag's advice and checked the voltages before buying another one but sometimes that works out too.
Good luck. Let us know how it worked out.
Of course he should have done with no doubt that c5 diag had suggested but the ls 2 tb is known for this exact problem. Yes he's taking the easy route and he might get lucky. But then if it doesn't work he can return. And last it usually is a problem with boosted cars.
Of course he should have done with no doubt that c5 diag had suggested but the ls 2 tb is known for this exact problem. Yes he's taking the easy route and he might get lucky. But then if it doesn't work he can return. And last it usually is a problem with boosted cars.
If the LS2 throttle body is problematic, what other throttle body should I go with to fix the issue?
If the LS2 throttle body is problematic, what other throttle body should I go with to fix the issue?
I know one member was fighting this issues for over a year his tuner finally said the hell with and put his stock ls1 back on and after returned his problem was gone he was around 1khp but he didnt care of the losing hp which he didn't even tell and he modify his LS2 intake to a 3 bolt adapter. And then many other changed ls2 to ls2 and got lucky and fixed the problem. But you have to get a tuner involved to make sure like c5 diag said that your in the right values.
I am having issues with unsteady RPMs that feel as if the car is "2-stepping" at about 1500-2000 RPMs and I am getting codes P1120, P1220 - Throttle Position Sensor 1 & 2 Circuit. Over the course of 100-150 miles of driving the problem worsened. The RPMs typically became unsteady at around 3000-4000 RPMs, and over the course of 100-150 miles of driving, the car is "2-stepping" around 1500-2000 RPMs (see attached YouTube link https://youtube.com/shorts/GoRyUYF-8fc ). I have a ls2 throttle body on my ls1 corvette along with the ls1 to ls2 throttle body harness adapter. From my understanding, the cause of the problem or multiple problems can be:
APP (accelerator pedal position) sensor
TPS (throttle position sensor)
TAC (throttle actuator control) module
Faulty wiring/connection
From my understanding, throttle position sensors 1 & 2 are both integrated into the construction of the throttle body. If this is the problem, I need to replace the ls2 throttle body for $500+ or perform "electrical surgery" on the ls2 throttle body. If the APP sensor is the problem, then I need to replace the pedal assembly for a fraction of the price of a ls2 throttle body (hopefully this is the problem). If the TAC module is the problem, then I need to replace the TAC. I'm not too sure which of these 3 items is problematic (or faulty wiring/connections).
I have attached a HPL file from my VCM scanner that shows my throttle position percentage and accelerator pedal position percentage. This test was with the ignition off and accessories on. When the accelerator pedal is fully depressed, my throttle position is at 100% and my accelerator pedal position is at 112%. How can my APP be above 100%? Is that normal? In the test file, I slowly depress the pedal, and if you pay attention to the throttle position percentage, as it reaches 20-30%, the throttle seems to not move linearly with the accelerator pedal position.
My car is tuned with an LSA blower and I'm not sure if my tuner changed parameters with the pedal position (see attached HPT file) or if changing APP parameters are even possible in HPT.
From this information, what is the problem? The TPS? The APP sensor? Or maybe the TAC?
How should I go about diagnosing the problem?
@Bill Curlee@Speedy007@whatyacallit@C5 Diag I hope you're all still active around these forums because you all gave some insightful information regarding this issue I had nearly 4 years ago (this is the same problem from a pervious thread 4 years ago but I narrowed the problem down).
P1120, P1220 - TP Sensor 1 & 2 Circuit Problem
