P0120 TP Sensor 1 Circuit Malfunction (With Electronic Throttle Body)

This test tutorial will help you diagnose a P0120: Throttle Position (TP) Sensor 1 Malfunction with the Throttle Actuator Control (TAC) Throttle Body in a step-by-step manner using only a multimeter.

In case you're wondering what the heck is TAC, it means that your pickup's engine has the GM electronic throttle body (pictured above) instead of a mechanical accelerator cable.

You can find this tutorial in Spanish here: Cómo Probar El Código P0120 (Sensor TPS 1) Del Cuerpo Del Acelerador Electrónico (at: autotecnico-online.com).

This tutorial complements these tutorials (which are also found here at easyautodiagnostics.com):

TPS Sensor

• GM Electronic Throttle Body Circuit Descriptions And Testing Tips.
• Cleaning The GM Electronic Throttle Body (4.8L, 5.3L, 6.0L).
• P0220 TP Sensor 2 Circuit Malfunction (With Electronic Throttle Body).
• Resistance Specifications Of The TAC Electronic Throttle Body.

APP Sensor

• How To Test The GM Accelerator Pedal Position (APP) Sensor 1.
• How To Test The GM Accelerator Pedal Position (APP) Sensor 2.
• Resistance Specifications Of APP Sensor 1, APP Sensor 2.

If your GM pickup (SUV or van) has the mechanical throttle body (with an accelerator cable) and you need to test the Throttle Position (TP) Sensor, then the article you may need is this one: TPS Test.

Symptoms Of A Bad TP Sensor 1

When the Throttle Position Sensor 1 goes bad, you're gonna' see the following symptoms:

1. Check engine light will be lit on your instrument cluster.
2. Diagnostic trouble code:
   • P0120:Throttle Position (TP) Sensor 1 Malfunction.
3. Lack of power when accelerating the engine.
4. The throttle position (TP) signal is less than 0.37 Volts or more than 4.51 Volts (for more than 1 second).
5. You'll see the message center display: ‘Reduced Engine Power’ (and of course the engine will operate in Reduced Engine Power mode).
6. The engine may be turning off (this is done by the PCM).

The one symptoms that you'll be able to test (with a multimeter and the help of this tutorial), is the throttle position (TP) signal described in number 4 above.

What Tools Do I Need?

A scan tool is a handy (and a must have) tool to read diagnostic trouble codes and view some of the Live Data parameters it provides, but you don't need one to test TP Sensor 1.

The most important tool that you're gonna' need is a multimeter. Your multimeter can either be a digital multimeter (with a bar graph) or an analog multimeter.

Here are some of my suggestions:

1. Analog multimeter or a digital multimeter with a bar graph.
   • Your digital multimeter must have a bar graph to accurately test TP Sensor 1 in Ohms mode.
   • The Fluke 115 digital multimeter has a bar graph. You can see an example of it here: Abe's Multimeter Recommendation.
   • You can use an analog multimeter too.
2. Scan tool.
   • As I mentioned above, you don't need to use a scan tool to take advantage of the test info in this tutorial, but it does come in handy to read any trouble codes.
   • If you don't own a scan tool yet, I recommend taking a look at these 2 articles I've written: Scan Tool Essentials You Should Know! (at: troubleshootmyvehicle.com) and Actron CP9580 Scan Tool Review (also at: troubleshootmyvehicle.com).
3. Basic hand tools
   • These are: ratchet wrench, sockets, etc. that you'll need to remove and replace the electronic throttle body if the Throttle Position Sensor 1 is bad.

Where To Buy The TAC Components

You can run down to your local auto parts house and buy a remanufactured throttle body for about US$165, or you can buy a new AC/Delco unit online and as you're already aware, AC/Delco is the factory original brand.

This is where you can buy the AC Delco throttle body, throttle body gasket (which needs to be installed without any type of sealant/glue) and the connector:

Circuit Descriptions Of The Electronic Throttle Body Connector

Throttle Position Sensor 1 is located inside the electronic throttle body on your pickup (SUV, van). As a matter of fact, the electronic throttle body is made up of 3 major components, and these are:

• Throttle Position Sensor 1.
• Throttle Position Sensor 2.
• TAC motor (this is the electric motor that opens and closes the throttle plate).

This means that the electronic throttle body's electrical connector has a total of 8 wires sticking out of it. 3 of those 8 wires belong to TP Sensor 1.

Specifically, each of those three TP Sensor 1 circuits feed or return a specific type of signal and these are:

1. A 5 Volt DC power circuit (this is known, in GM tech terms, as the 5 Volt Reference Circuit).
2. A Ground circuit (in GM tech terms, Ground is known as the Low Reference Circuit).
3. Throttle Position (TP) Signal circuit.

Below, you're gonna' find a table (chart) with the job description of each of the 8 wires that stick out of the electronic throttle body's electrical connector.

Electronic Throttle Body Connector Pin outs
Pin	Wire Color	Description
A	Yellow	TAC Motor Control 1
B	Black w/ White stripe	Low Reference (Sensor Ground - TP Sensor 2)
C	Brown	TAC Motor Control 2
D	Black	Low Reference (Sensor Ground - TP Sensor 1)
E	Light Blue w/ Black stripe	5 Volt Reference ( TP Sensor 2)
F	Purple	TP Sensor 2 Signal
G	Dark Green	TP Sensor 1 Signal
H	Grey	5 Volt Reference ( TP Sensor 1)

TEST 1: Throttle Body Connector Wiggle Test

The very first thing we need to do, is to make sure that the electronic throttle body connector's wires are not causing the problem. To be a bit more specific, one of the most common problems encountered with GM's electronic throttle body is one of the 8 wires (that stick out of the connector) breaking.

GM is well aware of this problem, since several folks (that I know personally) have gone to GM/Chevy dealership service departments to have a TAC issue/problem resolved and it was suggested to them (very emphatically) that the connector needed to be replaced whether it was good or not, but GM has NOT recalled the connector yet, so my buddies ended up paying for the connector (and labor to replace) out of their own pocket.

To go into even more detail, about what happens, the copper wire breaks inside the insulation, close to the point where the copper wire connects to the metal terminal (inside of the connector). What stinks about this (when it happens), is that the rubber insulation covering the copper wire WILL NOT break.

So, just looking at the wires, you'd never realize the copper wire is broken inside the insulation and causing an ‘open-circuit’ problem.

There are two things you can do to check this and they are:

1. Visually check to see if the wire is crimped excessively at that point (with the engine off).
2.     -and-
3. Wiggle the connector with the engine idling, to see if it wiggling it will cause a difference in the engine's idle.

The photo above is an actual example of a broken wire in the electronic throttle body's 8-wire connector. The wire was not broken (initially), I took the photo after a pulled on the wire and the rubber insulation broke.

OK, this is what you need to do:

PART 1

1. Start the engine and let it idle.
2. Grab the electronic throttle body wires and gently wiggle them.
3. When done with the wiggle test, turn off the engine.

PART 2

4. Disconnect the electronic throttle body's 8-wire connector.
5. Visually inspect each wire at the point where it comes out of the connector
   • What you're looking for is any abrupt bends in the wire (these abrupt bends are a tall-tale sign of a broken wire).
   • It's been my experience that the break in the wire has always been right near where the wire goes into the connector and so this is the area you need to focus on.

Let's interpret your test results:

CASE 1: None of the 8 wires of the connector are broken and if the engine's idle did not react (to the wiggle test). So far so good (since we don't have to worry about the wires).

The next step is to see if Throttle Position Sensor 1 is getting power. Go to: TEST 2: 5 Volt Reference Circuit Test.

CASE 2: One of the 8 wires of the connector is broken. This is not good. Replacing the connector with another will resolve this issue. To find out where to buy the connector and save some money, check out this section: Where To Buy The TAC Components.