Diagnosing & Fixing Fault Code P0121: Throttle Position Sensor on 2013 GMC Yukon XL

2013 GMC Yukon XL with Check Engine Light and Brake light on

My spousal unit arrived home and promptly showed me the picture taken of the dreaded CHECK ENGINE light on our 2013 Yukon. (The parking brake is set, so that’s not a trouble light, but rather an indicator.) Last time, about six months ago, it was an Oxygen Sensor… replaced it and it seemed to fix that problem.

I’m not a real mechanic. I’m a proud YouTube mechanic. haha. I have an old, first generation bluetooth OBDII code reader, and an old Samsung Galexy S III Mini android cell phone (cracked in multiple places) with Torque Lite OBD2 app. Somehow I manage to get them synced up and reading codes.  This time is shows P0121: Throttle/Pedal Position Sensor/Switch A Circuit Range /Performance Problem.  Of course the very first step is simply clear the faults. LOL.

Google knows how to fix a car.

Of course Google is my second step to quickly find out the complexity of diagnosis and repair.  I found this great site describing possible solutions, it’s got to be the best site for decoding OBDII.  I’ve got a volt meter and will start with that for step 1.  But the very next, step 2, mentions oscilloscope.  I don’t have one of those.  But I remember seeing them thirty years ago at Cleveland’s Amateur Electronic Supply Co., a salvage yard for old electronic parts.  By now, they must have lower cost units on Amazon.com.  And they do… I found this unit, a Quimat Digital Oscilloscope Kit, fully assembled for about $43.  OK, we’ll start with step 1 before ordering that.  I’ll be back to update my progress…

11/12/2019 – Of course the very first thing to do is simply reset the light. No harm, no foul. So I reset it a few days ago.  Driven each day.  Still has not come back on.

11/17/2019 – Check Engine Light just returned. My spousal unit said it seems to come on when the fuel is low… but I don’t think it has anything to do with it.  (A few years back, we did have an O2 sensor that seemed to error when old gas was run in car – but then we shouldn’t be using old gas!) Just ordered a Throttle Position Sensor (TPS) from Advance Auto Parts.  They usually have great online promotions, will ship to home.

Screenshot of part ordered online from Advance Auto Parts TPS Throttle Position Sensor

Replacing 2013 GMC Yukon XL Throttle Position Sensor

11/21/2019 – The new Throttle Position Sensor arrived from Advance Auto Parts.  I followed instructions on this video, Replace Throttle Position Sensor Tahoe, Silverado, and Suburban.  Not mine, but a great video… some comments…

at 1:09 depicts a gray tab. When reinstalling the connector, the small piece that you see being pushed in here (when it’s being taken off) with a screw driver broke off.  Here’s my picture with the tab, center piece broke off when removing according to video. (Also note the X-Treme Tape Silicone Rubber Self Fusing Tape that I replaced the worn electric tape that was already peeling off around the wires just before connector.)

Connector ACDelco Throttle Position Sensor (TPS) 19259452

at 3:14 shows new TPS being easily placed into position – after lining up tab.  There were two removable metal jumpers that remained in place on the throttle from the old sensor.  I didn’t notice them at first, and the new sensor wouldn’t fit properly until I removed/pulled the old jumpers out of place.  Here… the NEW sensor is on the LEFT, the OLD sensor is on the RIGHT… you’ll notice the old sensor doesn’t have the little brass tabs.  The new sensor on the left has two little brass tabs.  Just pull the old tabs off the throttle.

Old and new throttle position sensors

at 3:18 to 3:20 depicts the new clips being reinstalled in two seconds.  Of course it turns out the most difficult, tedious part of the entire job… is skipped over in two seconds. Haha.  Reinstalling the clips took a LOT of effort.  I got them all on, but it wasn’t easy.  I don’t have any advice, other than push real hard, and start with one side partially on, but the other side over the lip of the edge. In my vehicle, there was no room for the hammer that was suggested on video… or I would’ve used it.

at 3:21 depicts the gray tab, re-attached to the connector, and unbroken.  When I went to reinsert the gray tab, as noted above, the middle piece snapped off.  It still connects securely and seems to stay in place, that little tab looks like the ‘triple lock’, but there are two other mechanisms holding the plug in place.

Other than those comments, that’s a great video.  Thanks for helping me continue to persevere as a YouTube mechanic! haha

We’ll see if our friendly little CHECK ENGINE light comes on again!!

11/22/2019 So… the dreaded CHECK ENGINE light came on again… after doing some additional googling… apparently I didn’t do the ‘relearn’ process.  I did it, here’s a great post that showed me how https://ricksfreeautorepairadvice.com/gm-throttle-body-relearn-procedure/ …reset light.  Lets try again!

11/24/2019 AGAIN… CHECK ENGINE!! Wish I would’ve ran across this video before replacing the TPS…  THIS video shows how to CLEAN the throttle body, but even better, it shows you how to remove the throttle body – which also lets you easily access all those pesky clamps that took so much time to replace.  Also ran across this ‘official’ Technical Service Bulletin (TSB) about cleaning and relearning procedures for an older model.  Will remove, clean real good, clear code…  relearn…. lets see how it goes…

Cleaning 2013 GMC Yukon XL Throttle Body

11/26/2019 So today I removed the throttle body and gave it a good cleaning with carb cleaner.  It’s not perfect, because I didn’t use any brush or anything rougher than a clean rag.  But it’s much better.

Before Cleaning Original ACDelco Throttle Body Unit 217-3151After cleaning Original ACDelco Throttle Body Unit 217-3151 Before cleaning Original ACDelco Throttle Body Unit 217-3151After cleaning Original ACDelco Throttle Body Unit 217-3151

So after cleaning and replacing, it was challenging to get ‘relearn’ the Throttle Position Sensor.  The procedure was clearly outlined in the above TSB, and many other places online.  But… it didn’t seem to ‘work’.  The only thing that worked was driving the car around for a few days, doing many dozens or hundreds of accels above 45mph and decels. Check Engine light came on multiple times, showing ‘high idle’ a few times, even a ‘multiple misfire’ once (yes, I’m going to replace plugs and wires soon, they’re pretty old.)  It would idle high sporadically, but seemed to do it less and less over the course of several days.

Okay, she was pretty dirty, we’ll see if our friendly little CHECK ENGINE light comes on …again!!

Cleaning the 2013 GMC Yukon XL air filter

12/6/2019 So… the CHECK ENGINE light has remained off.  However… it occurred to me I neededK&N Air Filter really dirty to clean the air filter.  It had been a long, long time since the air filter was cleaned.  I can’t remember when!  It has a K&N filter, I put it in when I bought the car. So I bought a K&N Recharger Filter Care Service Kit.  I was a little disappointed, since I wanted the ‘clear’ oil, as advertised in that link, but when it came in, it was red dye.  Used it anyway because I needed to clean it right away. I followed the instructions precisely, low pressure water and all.  But it was way too dirty and even after using the cleaner several times, and water from back, lots of debris remained.  So I took the filter in the house and painstakingly, with great precision, picked out every single little piece I could get with tweezers, being careful not to damage the fibers.

Air filter bolt in plastic bagReplacing missing 2013 GMC Yukon XL air filter housing bolt

While removing and cleaning the air filter, I noticed one of the four bolts holding the air filter cover was missing.  I found a new one, but it wasn’t cheap.  It’s called a GM OEM Air Cleaner Intake-Upper Cover Bolt, part #11611199.  I’m sure replacing this will probably tighten up the air slightly.


Mass Airflow CensorClean 2013 GMC Yukon XL Mass Airflow Sensor

Also while cleaning the air filter, I noticed the Mass Airflow Sensor just sitting there.  It must be dirty as heck, it’s never been cleaned, for longer than the air filter.  And the air filter was probably almost clogged!!  Of course I did my research, and used proper Mass Airflow Sensor Cleaner.






Grown up in Cleveland, Ohio, USA., previously lived in Phoenix, Arizona and Tampa, Florida areas, and recently co-located to Asheville, North Carolina area. Trained as an Army MP, but bit of a renegade career wise. Phil has owned businesses in insurance, private investigations, surety bail bond industries, and has experience in computer network engineering, hospitality in-room advertising, asset recovery and repossession, and even a few years traveling most of the lower 48 states, Ontario and Northern Mexico as a commercial over the road truck driver. Career long entrepreneur, property investor and rental manager. Phil loves sharing his experiences of great food, travel, experiences and products.

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