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

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.
6. 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.

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.

TEST 2: 5 Volt Reference Circuit Test

The second thing we need to do now, is to make sure that Throttle Position (TP) Sensor 1 is getting power. This power is in the form of 5 Volts DC and is provided by the Throttle Actuator Control (TAC) Module.

The wire that you'll be testing is the gray wire labeled with the letter H in the photo below.

Alright, this is what you need to do:

1. Place your multimeter in Volts DC mode.
2. With the red multimeter test lead, probe the wire identified with the letter H in the photo below.
   • This is the Gray colored wire of the connector.
3. Ground the black multimeter test lead directly on the battery negative (-) terminal.
4. Have a helper turn the key ON, but don't crank or start the engine.
5. If all is OK, your multimeter should register about 5 Volts.
6. Let's interpret your test results below:

Let's take a look at what your test results mean:

CASE 1: Your multimeter registered 5 Volts. So far so good, since this test result lets you know that Throttle Position Sensor 1 is getting power.

The next step is to check that Throttle Position (TP) Sensor 1 is getting Ground. Go to: TEST 3.

CASE 2: Your multimeter DID NOT register 5 Volts. Double check your connections and make sure you're testing the correct wire.

If your multimeter still does not register 5 Volts in this circuit, then you've found the cause of the P0120 diagnostic trouble code (DTC). Repair the issue causing these missing 5 Volts, since without power TP Sensor 1 won't function.

Now, although it's beyond the scope of this tutorial to troubleshoot these missing 5 Volts, here are the two most like causes:

1. There's an ‘open-circuit’ problem on this wire between the electronic throttle body's connector and the Throttle Actuator Control (TAC) Module.
2. The Throttle Actuator Control (TAC) Module is bad.