Help with P0121 code? - Pontiac G8 Forum: G8 Forums -
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post #1 of 4 (permalink) Old 12-29-2016, 08:48 AM Thread Starter
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Help with P0121 code?

Hello, I have a 2009 V6 G8 and lately I've been getting this "P0121 code - throttle/pedal position sensor switch A circuit range performance problem" It puts the car in reduced power safety mode.
It only seems to happen when it's quite cold outside and when it first started it seemed to go away as soon as the car was warmed up and shut off/turned on. Now it is happening a couple of times per week and only seems to stop when the car is shut down for a few hours before restarting..
I don't see any issues with the plugs on the pedal or throttlebody and I reseated them - I assume it's a bad sensor but I'm not sure which one and it's so cold outside lately I'd like to fix it once
Using the Torque app on my phone I've watched the pedal sensor value and the TPS(manifold) value and they are definitely different, am I understanding correctly that that's what this code refers to?

Has anyone experienced this before? Should I just go with my gut and replace the accelerator pedal sensor and see what happens?

My old cars (like my old Grand Prix) had affordable service manuals that would walk you step-by-step through troubleshooting each code - most frustrating part of owning a G8 is not having access to stuff like that - the pdf's available online don't contain nearly enough info.

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post #2 of 4 (permalink) Old 12-29-2016, 10:35 AM
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Location: Fountain Valley California
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The Throttle body contains 2 position sensor circuits within it. An both systems have to produce the proper signal back to the ECM. P0121 says that sensor circuit #1 performance is not good. I'm pretty sure the only fix for that code (since the wiring to the TB looks OK), is a new Throttle Body assembly. The P/N shown for that Throttle Body is 12615503 (AC Delco # 217-3150), and you best get a new mounting/parting gasket to go along with it. GM p/n 12593360 (AC Delco # 40-5083). You can buy both at Amazon (if you want). The Throttle body goes for $90 an the gasket about $14.

Your throttle pedal also contains 2 pedal position sensor circuits. But they would report different codes, if it was the problem.
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post #3 of 4 (permalink) Old 12-29-2016, 10:44 AM Thread Starter
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Oh wow, so I was headed down a completely wrong path.. Thank you so much, I'll try the new throttle body from Amazon and hopefully get through the rest of the winter without this annoying issue. Is there any learning process I have to do after I replace it or is it just idle the car, shut it off, idle the car some more, shut it off?
Thanks again!
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post #4 of 4 (permalink) Old 12-29-2016, 11:27 AM
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They do have an *idle learn procedure* in the service manual. But they say you would have to clear all of those fault codes first, or the learn procedure *will not* take place. Plus (if your in a really cold area) the coolant an intake air *has* to be above 41 degrees also. After replacing the TB, turn the ignition on (engine off) for 60 seconds, then turn the ignition off. Turn the ignition back on (with engine off) an clear any fault codes that come up. I don't think that it is mandatory to do this. The Throttle Body is always learning the proper idle range it needs to be in. But this learn procedure probably puts it in the most correct setting wrat now.
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