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aj510

Need Help Please! Codes P0120 P0121 P1780

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SOMETIMES WHEN I START MY 1998 LS400 WITH 125,000 MILES ON IT THE VSC OFF AND ENGINE LIGHT COMES ON, ALSO WHEN I PUT IT IN DRIVE AND PUSH THE GAS PEDAL IT DOES NOT REACT SO I TURN OFF THE CAR AND START IT AGAIN TO SEE IF IT WILL MOVE THEN IT TAKES ME MORE THAN A FEW STARTS FOR THE CAR TO MOVE WHILE IN THE DRIVE POSITION. I DON'T KNOW IF MY BATTERY IS CAUSING IT TO DO THIS OR A SENSOR ETC. PLEASE SOMEONE HELP. THESE ARE THE CODES I FOUND MAYBE CAUSING THIS

P0120

P0121

P1780

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p1780 PNP failure. Seems the PNP switch is the Park Neutral Position switch which is used to sense the position of the shifter. Sometimes these switches may stick or if the car was started/stopped with the shifter in an "in between" position the computer may have seen it as a malfucntion.

I would suggest clearing the code and seeing if it comes back. If it comes back have the switch replaced. Most times these swicthes are on the side of the transmission.

p120 & 121 .. PO120 Throttle position Sensor Malfunction & PO121 Tp Sensor Fault .

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p1780 PNP failure. Seems the PNP switch is the Park Neutral Position switch which is used to sense the position of the shifter. Sometimes these switches may stick or if the car was started/stopped with the shifter in an "in between" position the computer may have seen it as a malfucntion.

I would suggest clearing the code and seeing if it comes back. If it comes back have the switch replaced. Most times these swicthes are on the side of the transmission.

p120 & 121 .. PO120 Throttle position Sensor Malfunction & PO121 Tp Sensor Fault .

I ERASE THE CODES AND THE LIGHT STILL COMES ON VSC OFF AND ENGINE LIGHT, YES YOU ARE RIGHT ABOUT THE SHIFTER SOMETIMES I HAVE TO WIGGLE IT TO GET THE COMPUTER OR SENSORS TO KNOW ITS IN DRIVE BECAUSE ON THE DASH THE "D" POSITION DOESN'T LIGHT UP GREEN LIKE ITS SUPPOSE TO. DO YOU THINK I CAN CHANGE THE SWITCH MY SELF OR DO I HAVE TO TAKE IT TO A SHOP? IF I CAN, CAN YOU POST ME SOME INFO HOW TO DIY, THANKS.

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aj510, welcome to the forum, but please, STOP USING ALL CAPS! Thank you.

Can you help me out then!? I am having problems with this vsc off cel crap when I'm in stop and go traffic it comes on then I'm in LIMP mode wont accelerate just idles slowly moving, i tried all the things people said tighten gas cap, disconnect battery for 20 minutes, etc. someone help because its dangerous when I'm at the stop light and stuck on limp driving like 2mph in busy traffic with kids in the car. Thanks In Advance

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Can you help me out then!? I am having problems with this vsc off cel crap when I'm in stop and go traffic it comes on then I'm in LIMP mode wont accelerate just idles slowly moving, i tried all the things people said tighten gas cap, disconnect battery for 20 minutes, etc. someone help because its dangerous when I'm at the stop light and stuck on limp driving like 2mph in busy traffic with kids in the car. Thanks In Advance

Billy already gave you the link. He's a professional mechanic with decades of experience. Don't ask for help again when its been given. Report back if it works or doesn't after you've tried it. Asking the same question over and over is the definition of ......

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Can you help me out then!? I am having problems with this vsc off cel crap when I'm in stop and go traffic it comes on then I'm in LIMP mode wont accelerate just idles slowly moving, i tried all the things people said tighten gas cap, disconnect battery for 20 minutes, etc. someone help because its dangerous when I'm at the stop light and stuck on limp driving like 2mph in busy traffic with kids in the car. Thanks In Advance

Billy already gave you the link. He's a professional mechanic with decades of experience. Don't ask for help again when its been given. Report back if it works or doesn't after you've tried it. Asking the same question over and over is the definition of ......

its not the same question. their was 3 codes the one he helped me with was a switch for the transmission if you read it. I don't understand i ask for help and then i get someone cheer leading for billy lol. well thanks curiousB for the reminder of the link for one of my 3 problems i guess I'll PM billy or lexmex

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So now I am still getting P0120 code which is TPS pedal position sensor!So with a Ohmmeter, I tested the RESISTANCE of TPS Sensor, the Throttle Control Motor, and the PPS Pedal Position Sensor, They ALL came out GOOD.

So I thought of cleaning the throttle body? Do you think if the Throttle body is dirty it will give me this P0120 code?

Or maybe its a ECM problem?

And I want to clean the Throttle Body anyway to just to try eliminate idle problems

So does anyone have PICTURES or VIDEOS of 1998-2000 LS400 Do It Yourself Tutorials? Because I read some people messing up sensors hoses etc. trying to clean the Throttle Body! :wacko:

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To test those sensors you have to check resistance across the full travel of the wiper. What you are looking for is a dead spot where the wiper gets no reading and the ECU logs that as an error. You can do this with a voltmeter and check the wiper (output) signal from the sensor while gradually moving the throttle plate by hand. If the meter moves steadily with your turning it is probably OK. If you hit spots where the meter jumps around and gets erratic that probably means a dead spot on the TPS and it needs to be replaced. Similar logic applies to PPS.

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To test those sensors you have to check resistance across the full travel of the wiper. What you are looking for is a dead spot where the wiper gets no reading and the ECU logs that as an error. You can do this with a voltmeter and check the wiper (output) signal from the sensor while gradually moving the throttle plate by hand. If the meter moves steadily with your turning it is probably OK. If you hit spots where the meter jumps around and gets erratic that probably means a dead spot on the TPS and it needs to be replaced. Similar logic applies to PPS.

So do I test this with the car running or off or it doesn't matter?

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When we repaired my starter. I cleaned my TC valve, it was not very dirty (141k), see photo, so I just sprayed carb cleaner in it and it came out clean. no problems so far.

post-5365-128330808865_thumb.jpg

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I think if key is on but engine not running the sensor should have voltage across it. You can then measure from ground to wiper of the sensor and see if output moves steady as you move throttle. You are just looking for steady climb of voltage as you turn. Jumpy readings indicate the problem...

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I think if key is on but engine not running the sensor should have voltage across it. You can then measure from ground to wiper of the sensor and see if output moves steady as you move throttle. You are just looking for steady climb of voltage as you turn. Jumpy readings indicate the problem...

thanks so i tested the output it was erratic on the TPS sensor so now i will change it hope it fixes it :blink:

but do you know how to adjust it when I put the New TPS on like some DIY tutorials, all the DIYs I see are early 90s LS400 different Engine?

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I looked in my service manual '04 but it didn't mention any calibration/alignment procedure. It might be self discovering in the ECU software where it figures out the high and low readings as it sees inputs and then goes from there. Not 100% sure on this one.

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