When a trouble code P0121: Throttle Position Sensor Performance Problem lights up the check engine light on your Chevrolet Silverado or GMC Sierra, you can usually conclude that the throttle position sensor has failed.
In this tutorial, I'll explain what this trouble code means. I'll also talk about some of the tests you can perform to find its cause and solution.
Contents of this tutorial:
You can find this tutorial in Spanish here: Código P0121 ¿Qué Significa? (1999-2002 V8 Silverado, Sierra, Suburban, Tahoe, Yukon) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
Chevrolet:
- Silverado (1500, 2500):
- 4.8L, 5.3L: 1999, 2000, 2001, 2002
- Silverado (2500):
- 6.0L: 1999, 2000, 2001, 2002
- Suburban (1500):
- 5.3L: 2000, 2001, 2002
- Suburban (2500):
- 6.0L: 2000, 2001, 2002
- Tahoe:
- 4.8L, 5.3L: 2000, 2001, 2002
GMC:
- Sierra (1500, 2500):
- 4.8L, 5.3L: 1999, 2000, 2001, 2002
- Sierra (2500):
- 6.0L: 1999, 2000, 2001, 2002
- Yukon:
- 4.8L, 5.3L: 2000, 2001, 2002
- Yukon XL (1500):
- 5.3L: 2000, 2001, 2002
- Yukon XL (2500):
- 6.0L: 2000, 2001, 2002
RELATED TROUBLE CODES:
- P0122 -What Does It Mean? (1999-2002 V8 Silverado, Sierra, Suburban, Tahoe, Yukon).
- P0123 -What Does It Mean? (1999-2002 V8 Silverado, Sierra, Suburban, Tahoe, Yukon).
What Does Trouble Code P0121 Mean?
A P0121: Throttle Position Sensor Performance Problem tells you that the throttle plate angle the TPS is reporting to the fuel injection computer does not correspond to what the actual throttle plate angle is.
This is usually because the throttle position sensor is:
- Reporting a lower than expected voltage signal.
- Reporting a higher than expected voltage signal.
- Reporting a voltage signal that's stuck in one value.
Common Symptoms Of A P0121 Trouble Code
The throttle position sensor is one of several key input engine management sensors the fuel injection computer uses for fuel injection calculations.
Since the TPS is a critical component of the engine management system, when it fails, engine performance will suffer.
You'll see one or more of the following symptoms:
- Rough or low idle.
- Very high idle.
- Engine may start and stall.
- Little to no acceleration
What Does The Throttle Position Sensor Do?
As you're already aware, the accelerator pedal is connected to the throttle plate via an accelerator cable.
As you step on or off the accelerator pedal, the throttle plate opens or closes to admit more less air into the engine.
It's the throttle position sensor's job is to inform the fuel injection computer the throttle plate's angle as it opens/closes.
When the throttle plate is in its closed position (like when the engine is idling and your foot is off the accelerator pedal), the TPS signal voltage is about 0.4 to 0.9 Volts DC.
Now, as the throttle plate opens, the throttle position sensor signal voltage increases. At wide open throttle (WOT), the TPS signal voltage is around 4.5 Volts.
As the throttle plate closes, the throttle position sensor signal voltage decreases.
As long as the fuel injection computer sees the TPS voltage signal increasing/decreasing, it knows you're stepping on/off the accelerator pedal (and that the TPS is functioning correctly).
What Causes A P0121 Trouble Code?
Generally a bad throttle position sensor is the cause of a P0121 trouble code, but not always.
I've also seen a bad MAP sensor cause this trouble code (since both the MAP and TPS share the same 5 Volt DC power circuit).
Other things that can cause a P0121 trouble code are:
- Bad MAP sensor (specifically, a MAP sensor that has an internal short-circuit problem causing the TPS to not receive 5 Volts).
- An open-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
- A short-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
- A bad TPS connector.
- TPS not receiving 5 Volts due to an open-circuit or short-circuit problem in its 5 Volt wire.
- Bad fuel injection computer (although very rare).
How To Diagnose And Repair A P0121 Trouble Code
To diagnose and repair a P0121 trouble code you'll need to test the throttle position sensor with the MAP sensor disconnected from its electrical connector.
The throttle position sensor test will check that the TPS signal voltage increases/decreases as you manually open/close the throttle plate.
You'll also need to make sure that the throttle position sensor is getting 5 Volts and Ground from the fuel injection computer.
If the TPS signal voltage DOES NOT increase as you open/close the throttle plate and the sensor is getting 5 Volts and Ground, then you can conclude that the sensor is bad and needs to be replaced.
If the TPS is good, then the next step is to test the MAP sensor.
You can find the TPS test explained in detail here: How To Test The TPS (1999-2002 V8 Chevrolet Silverado, GMC Sierra).
Where To Buy The TPS And Save
The following links will help you comparison shop for the throttle position sensor. The AC-Delco brand is the original equipment brand for all Chevrolet and GMC vehicles:
More Silverado, Sierra, Suburban, Tahoe, And Yukon Tutorials
You can find a complete list of V8 Silverado, Sierra, Suburban, Tahoe, and Yukon tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- Resistance Specifications Of The TAC Electronic Throttle Body.
- How To Test The GM Accelerator Pedal Position (APP) Sensor 2.
- How To Test The MAF Sensor (GM 4.8L, 5.3L, 6.0L, 8.1L).
- Blown Head Gasket Test (GM 4.8L, 5.3L, 6.0L).
If this info saved the day, buy me a beer!
