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Old 01-18-2013, 09:02 PM   #1 (permalink)
ChevyHDGert
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P0121 & P2135 Codes - Throttle Position Sensor

2011 GMC Sierra 2500 HD 4x4 6.0L

Check engine light came on today but it sometimes stays off after restarting engine.

Auto parts store came up with the above codes but they tell me that I have to replace the entire throttle body unit which includes the sensor.

Why can't I just buy the sensor? Or should I not worry about it since the light doesn't come on all the time?

I also had service stabilitrack and service traction control come up on display and my truck went into reduced power mode. So I'm not sure if that's a separate issue or related to those other codes. But the stabilitrack may have reset itself cuz it hasn't come on since that first time.
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Old 01-20-2013, 09:00 AM   #2 (permalink)
Manic Mechanic
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2011 GMC Sierra 2500 HD 4x4 6.0L

Check engine light came on today but it sometimes stays off after restarting engine.

Then it's intermittent, try cleaning the throttle with throttle cleaner and an old toothbrush. Make sure the key is off during the procedure to keep all fingers attached to hand. A throttle that is sticking on accumulated gunk is the most common cause of this.

Auto parts store came up with the above codes but they tell me that I have to replace the entire throttle body unit which includes the sensor.

Only if cleaning it doesn't solve the problem or it's not something else entirely causing the problem like a broken wire, bad connector, loose or corroded ground, shorted harness etc. Auto parts clerks are not auto techs as they may know about the parts but they rarely know what you actually need to do.

Why can't I just buy the sensor? Or should I not worry about it since the light doesn't come on all the time?

Because the unit is a very precise throttle angle controlling machine that supplies the control module with two counter acting throttle signals that allow the PCM to double check position at all times. It also has a powerful electric motor that actuates the throttle blade at all times when the key is on. Since this is a very important control of vehicle movement it must be very accurate. For efficiency of physical size and cost to produce it is compactly integrated. It is also well sealed from the environment to provide long term operation. If it was just a bunch of user accessible parts screwed together it would not be nearly as good at any of these things and cost a lot more to boot. They cost enough as it is, be glad they work as well as they do and don't cost more and take up a cubic foot of under hood space on a Corvette.
You can not worry about it but it will just get worse and make driving more troublesome and have greater potential to put yourself and others in peril should it go into reduced power just as you need it to avoid a collision or get a boat out of the the lake etc.

I also had service stabilitrack and service traction control come up on display and my truck went into reduced power mode. So I'm not sure if that's a separate issue or related to those other codes. But the stabilitrack may have reset itself cuz it hasn't come on since that first time.

It is directly related to the confusion over throttle position. You do know that your right foot is only mashing two other sensors telling the PCM what you want it to do right? If not start thinking about it that way, new cars and trucks are not so much gas, gears, and brakes as they are a computer network that moves people and stuff around by controlling the gas, gears, and brakes. So if the computers can't figure out what they are really doing mechanically because they don't know for sure where the throttle is at any given moment it stops trying to control skids and roll-overs and even driving straight down the road at 35 mph and just reverts to idle and creep in first as the only two possibilities for the user to operate the vehicle. The stabilitrack, traction control, ABS, engine, transmission, and all other control module computers dependent upon accurate control of the throttle position will turn the warning light on to let you know they are no longer working for you. Reduced Power is the PCM no longer trying to control normal engine operation but allowing it to stay running and barely roll so as to still provide some obligatory functions such as heat in a blizzard or power for lights in the dark and to get to shelter or repairs eventually.

Long story short, try cleaning it. Physical maladies such as gunk sticking to the throttle blade and bore cause all kinds of computer problems. The auto parts company knows that if you put a new one on it will most likely solve the problem and make them the most money. At least the brake pedal still pushes down a hydraulic cylinder connected to the actual brakes.

Vernon
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Last edited by Manic Mechanic; 01-20-2013 at 09:05 AM.
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