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

OBD II trouble code P1121 TP Sensor Signal Lower Than Expected is telling you that the throttle position sensor (TPS) is reporting a continuous low throttle plate angle when other sensor input (to the PCM) indicates the contrary.

Testing the throttle position sensor (TPS) to see if it has failed and causing the P1121 DTC is not hard. In this tutorial, I'm gonna' show you how troubleshoot the throttle position sensor (TPS) in a step-by-step way.

P1121 Basics You Need To Know

The throttle position sensor's function is to measure the amount that the throttle plate opens and closes (throttle angle). It then sends this info to the PCM (Powertrain Control Module = Fuel Injection Computer) as a volt DC signal.

The PCM then uses this throttle angle information to either inject more or less fuel and a host of other things to keep your Honda running and performing optimally.

To give you a few more specifics:

1. As you step on the accelerator pedal,
   • The throttle plate opens and the TP sensor measures how much and relays this to the PCM.
   • The fuel injection computer injects more fuel.
2. As you let your foot off the accelerator pedal,
   • The throttle plate closes and the TP sensor measures how much and relays this to the PCM.
   • The fuel injection computer injects less fuel.

This makes the throttle plate angle information a critical part of the computer's fuel injection, ignition system, automatic transmission control strategy (to name a few).

Now, when the PCM sees the TP sensor producing a low voltage that indicates a closed throttle plate even though other sensor inputs indicate otherwise, it sets a code P1121 TP Sensor Signal Lower Than Expected and lights up the check engine light (CEL) on your Honda's instrument cluster.

How The Throttle Position Sensor (TPS) Works

As you're already aware, your Honda's throttle position sensor is a 3-wire sensor.

Each wire has a specific job to do. By this, I mean each wire carries a specific type of signal to or from the TPS to the PCM.

To better understand how we're gonna' test the throttle position sensor (TPS), in this tutorial, I'm going to briefly describe each wire's job and how the sensor works.

Don't worry, it's nothing too technical and it's all in plain English:

1. The TP sensor is a 3 wire sensor.
   1. Wire labeled with the number 1.
      • Feeds Ground to the TP sensor.
      • Ground is provided by the PCM (internally).
   2. Wire labeled with the number 2.
      • Feeds the throttle angle voltage signal to the PCM.
      • This voltage signal varies depending on the amount of throttle plate opening.
   3. Wire labeled with the number 3.
      • Feeds power to the TP sensor.
      • In the form of 5 Volts DC and is supplied only with Key On Engine Off (KOEO) or Key On Engine Running (KOER).
      • Power comes directly from the PCM.
2. The TP sensor is a potentiometer. Its resistance changes in response to changes in the throttle plate's angle.
   1. With throttle closed, a small voltage is create and sent to the PCM.
      • At closed throttle the TP sensor outputs about 0.5 Volts DC.
   2. With throttle open to wide open, a bigger voltage is created and sent to the PCM.
      • At wide open throttle the TP sensor outputs about 4.5 Volts DC.

REMEMBER: The throttle position sensor (TPS), at closed throttle, produces a low voltage signal of around 0.5 Volts DC. As the throttle plate starts to open (as you step on the accelerator pedal and accelerate the engine), this 0.5 Volt signal starts to increase. At wide open throttle, the TP sensor will output about 4.5 Volts DC.

With this bit of information, let's move on to the next subheading.

Symptoms Of A P1121 Diagnostic Trouble Code

Since your Honda's PCM depends on the throttle position sensor for throttle plate angle information, when it receives a TP signal that doesn't square with actual engine operating conditions, your Honda's engine is just not gonna' run right.

You'll see one or more of the following symptoms:

1. Check engine light (CEL) shining nice and bright.
2. DTC P1121 is present.
3. Your Honda fails the state mandated emissions test.
4. Bad gas mileage.
5. Hard start and/or extended cranking time (after shut off).
6. Black smoke coming out of the tailpipe.
7. Hesitation when accelerating your Honda.

Let's find out what are the common causes of a P1121 DTC, in the next subheading.

Common Causes Of A P1121 Trouble Code

The 3 most common causes of trouble code P1121 are:

1. A bad throttle position sensor (TPS).
2. A broken TP sensor connector.
3. A problem in the sensor 3 wires. Specifically, a short in one of them.

Although extremely rare for this to happen, a bad PCM can also cause a false P1121 trouble code.

In this tutorial, I'll help you troubleshoot all three of the above. With this basic info under our belts, let's turn the page and get testing!

Where To Buy Your TP Sensor And Save

The Honda service manual tells you to replace the entire throttle body when the TP sensor fails but you don't have to.

The TP sensor is sold separately and can be installed as a stand-alone part.

Where can you buy the TP sensor? You can buy it at your local auto parts store but it's gonna' cost a whole lot more. I suggest taking a look at the price of the TP sensor in the following link and compare: Dorman 911-753 Acura/Honda Throttle Position Sensor.

Not sure the TP sensor listed fits your particular Honda? Don't worry, they'll make sure it fits your Honda, once you get to the TP sensor site, or they'll find the right one for you.