How To Test: P1121 OBD II Trouble Code (Honda 2.2L, 2.3L)

START HERE: Troubleshooting DTC P1121

The key to diagnosing the P1121 TP Sensor Signal Lower Than Expected OBD II diagnostic trouble code (DTC), that's lighting up your check engine light, is keeping in mind that the throttle position sensor (TPS) is stuck producing a low voltage signal.

If you recall, from the previous page, the TP sensor's voltage signal increases when the throttle plate opens and decreases when the throttle plate closes. So when a DTC P1121 is registered, the PCM sees the TP sensor reporting a low throttle angle when it knows, via other sensor inputs, that it should be reporting a higher voltage signal.

By manually testing the TP sensor with a multimeter (while it's still on the throttle body and connected to its electrical connector), you and I can find out if it's truly fried or not.

Here's a brief description of what you'll be doing test-wise:

1. Check the TP sensor's voltage signal with a multimeter.
   • You'll be connecting your multimeter to the TP sensor connector's middle wire and then manually opening and closing the throttle to see if the TP sensor produces a varying DC voltage signal.
   • TEST 1: Testing The TPS Voltage Signal.
2. Verify that the TP sensor is getting power (if TEST 1 is failed).
   • This is a simple multimeter test too.
   • TEST 2: Verifying The TPS Has Power.
3. Verify that the TP sensor is getting Ground (if TEST 1 is failed).
   • This is a simple multimeter test too.
   • TEST 3: Verifying The TPS Has Ground.

TEST 1: Testing The TPS Voltage Signal

Using a multimeter, the first thing we'll do is check to see if the TP sensor can create and report a variable voltage signal.

We'll attach the multimeter to the middle wire (circuit 2 in the image viewer) of the TP sensor and then manually rotate the throttle. Our multimeter should now record the TP signal voltage changes.

I'll explain everything in a step-by-step manner in the following instructions.

NOTE: To ensure the accuracy of your test, it's best to test the throttle position sensor (TPS) with the engine warmed up.

IMPORTANT: The throttle position sensor has to remain connected to its connector for this test to work (this is where a wire piercing probe comes in handy to get to the signal inside the wire. To see what one looks like and where to buy one, click here: Wire Piercing Probe Tool Review.

OK, let's start:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   Connect the red multimeter test lead to the wire labeled with the number 2 of the 3-wire TPS electrical connector (see the illustration above).
   
   The wire labeled with the number 2 is the middle wire of the 3-wire TPS electrical connector.
   
   NOTE: The TPS must remain connected to its connector to test the TPS voltage signal.

3. 3

   Connect the black multimeter test lead directly on the battery negative (-) post.

4. 4

   Turn the key to the ON position but don't crank or start the engine.

5. 5

   At this point, your multimeter should read between 0.4 to 0.9 Volts DC.

6. 6

   Manually rotate the throttle plate.
   
   NOTE: You'll get the best results by opening and closing the throttle plate directly on the throttle body instead of stepping on the accelerator pedal.

7. 7

   The multimeter should show an increasing voltage as you (or your helper) open up the throttle plate.
   
   Once the throttle plate is wide open, your multimeter should read somewhere between 3.5 to 4.9 Volts DC.

8. 8

   The multimeter should show a decreasing voltage as you begin to close the throttle plate.
   
   Once the throttle plate is fully closed, your multimeter should report the voltage you saw in step 5.

9. 9

   Using a screwdriver's handle, gently tap the TP sensor as you open and close the throttle plate and observe the multimeter.
   
   The purpose (of tapping the TP sensor with the screwdriver's handle) is to see if the TP sensor shows gaps in the voltage signal. Why? Because a good TP sensor will show a continuous increasing or decreasing voltage signal even while getting tapped by the screw-driver's handle.

Let's take a look at your test results:

CASE 1: The multimeter registered a smooth increase or decrease in voltage with no gaps. This is the correct and expected test result. In other words, the TP sensor passed with flying colors.

This test result also let's you know that the problem causing the P1121 trouble code is intermittent and not present at this point in time.

CASE 2: The multimeter DID NOT register a smooth increase or decrease in voltage, and you saw the voltage reading skip or go dead when tapping the TPS, then this means that the throttle position sensor (TPS) is bad. Replace the throttle position sensor.

Go to: Where To Buy Your TP Sensor And Save.

CASE 3: The multimeter DID NOT register any voltage. This test result doesn't condemn the TPS as bad just yet. Why?

Because the TPS may be missing either power or Ground. So the next step is to check that the TPS is getting power, go to: TEST 2: Verifying The TPS Has Power.