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Old 03-04-2013, 11:01 AM   #1
lateulade
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Default How to resolve error P2128

My wife's 06' Ion (85k miles) has started going into reduced power mode this week. Took it to AutoZone, and it's throwing code P2128, which I understand is the throttle position sensor. I've done a bit of searching, and some say to replace the accelerator (as the sensor is built in), others say it's likely a wiring issue.

I'm fairly mechanically competent, but would prefer not to mess with wiring if at all possible.

Is there any way other than parts swapping to determine the fix? I'd prefer not to have to take it to the dealer, but if I have to, I have to.

ETA: Had a similar issue about 6 months ago, and replaced the ECM / spark plugs, which resolved it until last week.

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Old 03-04-2013, 03:05 PM   #2
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Default Re: How to resolve error P2128

Your drive-by-wire system is all electronic; the pedal, throttle and pcm are all electronically integrated and wiring ties them together. As an emissions error code, this may qualify for warranty repairs. Call your local GM dealer or the 800 number for customer service for coverage. If not then there are choices to make. Bring it to GM for diagnosing and any repair or try some suggestions.

Search past threads about either P2128 issues in Ions or about the electronic pedal. One thread was about moisture possibly working its way into the pedal electronics and removing moisture the cure, from a leaking HVAC problem (condensation). The pedal sensor contains two feedback potentiometers, dual variable resistors used for signaling the pcm and for safety preventing a runaway engine - the reason for reduced power mode or more commonly known as 'limp home mode', only for true dbw systems. Visually check wiring and connections. Disconnect and reconnect the pedal wire harness to exercise the male/female terminals against any corrosion. If you are inclined, remove the pedal sensor and open it (if its not sealed/riveted against opening) carefully to examine it.

Only one member here in the Vue forums replaced his throttle actuator control (TAC) module with a used ebay one. His father opened the original tac and saw parts that may be serviceable with some cleaning. He recalled seeing similar parts while in the Air Force as an aviation technician.

Tac's and accelerator pedal position (APP) modules aren't DIY repairs with "no user serviceable parts" unless you're familiar with electronic parts like variable pots. The worst case scenario after messing around with this pedal is failure, reduced power mode requiring replacement. Best case scenario is finding out if whatever you do works and eliminates reduced power mode. Practice examining one on a used one if you like.

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Old 03-06-2013, 01:02 PM   #3
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Default Re: How to resolve error P2128

fdryer,

Thanks for your help! Sorry to be such a dummy -- but in this case, would I swap out the pedal, or the throttle body (which, from what I can see online, contains the sensor)? I ask because the throttle body is pretty inexpensive at about $75, and the Haynes manual makes it look like a snap to replace.

Thanks!

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Old 03-06-2013, 01:46 PM   #4
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Default Re: How to resolve error P2128

While swapping parts is the easy part, there are some EFI system parts that may require an electronic synching. Some terms used, depending on the part; recalibration, relearning, programming. For drive-by-wire parts like the throttle, a recalibration may be needed for the pcm to learn and calibrate a replacement tac module. I think the pedal is a simple swap. Trying the simple things should come first like 'exercising' the connections - disconnecting and reconnecting that may help improve the electrical connections on oxidized terminals. Look at the throttle plate movement when someone turns the ignition switch ON and exercises the pedal. Movement should be in direct proportion to pedal travel. Calibrating throttle is done by leaving the ignition switch ON (engine off) beyond 30-seconds when the pcm invokes a recal procedure of moving the throttle plate and checking throttle position against a stored reference. All taking less than 1-minute.

You can see parts from places like http://rockauto.com, http://parts.nalleygmc.com/, ebay, etc..

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Old 03-07-2013, 10:38 AM   #5
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Default Re: How to resolve error P2128

Thanks again! I'm going to try plugging and unplugging the cable (and making sure the connectors are clean).

Sorry for the n00b question, but should that not resolve the issue, the solution should be to just replace the accelerator pedal position sensor via the steps in the Haynes manual (6-8), correct?

Thanks!

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Old 03-07-2013, 12:22 PM   #6
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Default Re: How to resolve error P2128

Although the error code points to the pedal sensor, I would try the disconnect/connect for the pedal and then clean the throttle before resorting to replacing the pedal sensor. Since the throttle is maintenance free, cleaning won't hurt and worth trying as it is part of the dbw system. Both throttle and pedal have dual feedback sensors and outside of possible wear of these sensors (not likely with only 8xk miles), cleaning may help.

The throttle is spring loaded to close the throttle plate whenever power/ignition is OFF so propping the throttle open with fingers is needed when cleaning the bore.

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