In three simple test steps, you can find out if the TPS sensor on your GM car or pick up or SUV or Van is bad. You don't need a scan tool, since all three of the tests explained in this article are done with a simple multimeter. Every test has step by step instructions that will make diagnosing the TPS on your GM vehicle a breeze.
At the bottom of this page you'll find a complete list of GM cars, pick ups, SUVs and Vans this test article applies to. Also, in this section you'll find the info you'll need to successfully test and troubleshoot your TPS sensor or code.
You can find this tutorial in Spanish here: Cómo Probar el Sensor TPS (1996-2004 GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).
Symptoms Of A Bad Throttle Position Sensor (TPS)
Beside the check engine light (CEL) being on and driving you nuts, your Chevy or GMC pick up (or SUV or van) may be experiencing one or several of the following symptoms:
- TPS diagnostic trouble codes (DTC's) stored in the vehicle's computer's memory.
- P0121: Throttle Position Sensor Performance.
- P0122: Throttle Position Sensor Circuit Low Voltage.
- P0123: Throttle Position Sensor Circuit High Voltage.
- Really bad gas mileage.
- Transmission does not shift out of second gear.
- No power as you accelerate the vehicle.
- Hesitation when you step on the accelerator pedal.
What Tools Do I Need?
You don't need any expensive testing equipment to test the TPS on GM vehicle. You will need:
- A multimeter (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
- A helper.
- A repair manual for any remove and replace info you may need that this article does not cover.
Circuit Descriptions Of The GM TPS Sensor
Like the majority of throttle position sensors on modern fuel injected cars and trucks nowadays, the GM TPS is a three wire sensor. Each wire provides a specific type of signal to the TPS or to the PCM (Powertrain Control Module=Fuel Injection Computer). Below are the circuit descriptions of each:
- Circuit labeled 1:
- Throttle Position (TP) Signal Circuit. Blue wire.
- Circuit labeled 2:
- Ground Circuit (Provided by PCM internally). Black wire
- Circuit labeled 3:
- Power Circuit (5 Volts from PCM). Gray wire.
I'm gonna' make one very important recommendation to you when testing the signals of each wire. And that is to not probe the front of the female terminal of the TPS connector. It's best to use a tool to pierce the wire, such as a wire-piercing probe (you can see what one looks like here: Wire Piercing Probe) to get to and test the signal.
One last observation, these three circuits go directly to the PCM. So you need to be careful not to short these wires to power (12 Volts) or you may fry the PCM.
How Does The GM TPS Sensor Work?
Knowing how the throttle position sensor works will help you to understand the ‘why’ and ‘how’ of the three tests you're gonna' do in this article. So, in a nutshell when you crank and start your GM car or truck or SUV:
The throttle position sensor gets 5 Volts from the Gray wire and Ground from the Black wire. Both of these are provided internally from the PCM.
Once the car or truck starts and is idling and since the throttle is closed, the TPS sends a base throttle position signal of about 0.5 to 0.9 Volts to the PCM on the Blue wire (this voltage will vary slightly on your vehicle but should be within the 0.5 to 0.9 Volts DC).
When you step on the accelerator pedal to get the car or truck moving, the throttle opens causing the TP sensor to increase its voltage signal.
This is how the PCM knows that the throttle just opened up and that it needs to start injecting more fuel and advancing ignition timing, transmission shift points, etc.
As you let go off the gas pedal (accelerator pedal) the throttle closes and the TP sensor decreases the voltage signal to the PCM and returns to its base voltage signal, till you step on the accelerator pedal to move the vehicle and the cycle begins again.
Pretty simple stuff, no? Well testing it is just as simple. Since you'll be working in the engine compartment, no need to tell you (but I'm gonna' tell you anyway) to be alert and be very careful. Use common sense and take all necessary safety precautions. OK, let's get this show on the road, go to: TEST 1: Testing The 5 Volt Reference Signal.