The P0121 Honda OBDII Diagnostic Trouble Code occurs when your Honda Civic D16 ECU receives a throttle position signal that’s lower than what it expects. The throttle position sensor (TPS) is a huge part of how the Powertrain Control Module and how it controls fuel and spark in relation to closed loop operation, this trouble code can put your car into limp mode.

The P0121 Honda check engine code can cause serious engine operation issues and hinder your gas mileage and efficiency. Because your ECU does not know exactly what the throttle blade is doing or how much throttle is being fed, it won’t know how to run your car.

Limp mode exists so that you can take your vehicle into a mechanic or service to resolve the OBDII trouble code for your throttle position sensor. I’ll be showing you how to test the TPS in a 1997 Honda Civic EX with a D16Y8 motor. You will also learn how to test the sensor for power and ground, and test the range of the voltage output from the TPS signal wire.

This writeup is also pretty handy when you are installing an upgraded throttle body, or replacing the throttle position sensor and have to calibrate it for proper use in your Honda.

If you need to remove your TPS for any reason this writeup can also save you a lot of hassle and money, because Honda does not sell this sensor separately. They want you to purchase an entire throttle body assembly, a far more expensive option than a 50-60 dollar sensor.

What is the Throttle Position Sensor?

---

The TPS sensor is basically a three wire potentiometer that measures the amount the throttle body is being opened. This signal is converted to a signal that your Honda D16Y8 P2P ECU can recognize and use to operate closed loop mode.

Using the throttle position sensor signal as well as other important engine references, the P2P ECU in your Honda Civic “aims” for a exhaust air/fuel ratio of 14.7:1.

That’s roughly 14 parts fuel for one part air that is considered automotive “stoich“, which is what closed loop operation or closed loop mode is for. Without this important reference of throttle blade position, the job of running your engine efficiently becomes almost impossible.

What causes the P0121 Honda DTC Code?

---

Your P2P or similarly chipped VTEC ECU has received a reference signal from your TPS that’s much lower than what it expected. Either your throttle position sensor has gone bad or there are problems with the power and ground to the sensor.

There could also be a short or a break in the wire between your ECU and the TPS itself that causes your P0121 Honda code. When this occurs, you’ll need to replace the harness or repair the break in the ECU wiring.

How To Test your Civic Throttle Position Sensor – Fix your P0121 Honda Code

---

I will be breaking out the procedure on how to test your Honda Civic Throttle Position Sensor into four different steps, starting below. When your P0121 Honda trouble code is triggered, you can use this guide to test and erase your check engine light.

As usual, any modifications or testing you perform on your own car are not my responsibility. If you don’t feel like this is a job you can perform, take your car to an experienced auto shop.

Unclip the top half of the factory Honda airbox by undoing the metal band clips that hold the upper cover to the lower assembly.

Now you will need to disconnect the Air Intake Temperature sensor that’s mounted in your factory airbox.

Now pull up on the upper cover to remove and expose the air filter underneath the top of the airbox. Remove your factory filter to reveal the throttle body and complete lower airbox assembly.

There’s going to be one 8mm bolt that holds the left side ( passenger side ) of the lower assembly in place, remove this bolt now.

Disconnect the vacuum hoses that are connected to the front of the airbox, and now you are ready to pull up and away on the lower assembly.

Here’s a look at the intake manifold and throttle body opening, you are now ready to move onto the first part of the testing procedure.

Checking for TPS Power

---

Take some time to familiarize yourself with the throttle position sensor and the three pin weatherproof connector and wires. Put the key to the “ON” position but do not turn the engine on, your sensor should now be powered.

Take your voltmeter and put the black lead to the battery ground, now put the red lead to the third pin or PIN 3. This yellow wire with red stripe should be sending you 5 volts of switched power and is needed for your Honda D16 TPS to work properly.

Now switch the red lead to the battery terminal and put the black lead to first pin or PIN 1. This green wire with black stripe should have a ground present, which will allow your TPS to receive power.

Checking for TPS Signal

---

It’s the moment of truth or at least for you DIY’er, the moment where you find what the source of your P0121 trouble code is. Take your voltmeter and secure the black lead to the negative terminal on your battery. Now with one hand backprobe the middle red wire with black stripe for the signal from your throttle position sensor.

You should see a voltage values here, which means you are ready to test the operation of your throttle position sensor. Using your other hand, deflect or push down on the throttle body blade to manually open your throttle body from the engine bay.

This testing method is the best and easiest way to determine what next steps need to be taken to resolve your P0121 DTC trouble code. Using your hands to manually open your throttle blade, the voltage reading on your multimeter should range from less than one volt all the way to 3.6 to 4.8 volts DC. Make sure you have between the 3.6-4.8 voltage range at Wide Open Throttle (WOT) and less than one volt with the throttle blade closed.

Now slowly open and close the throttle, going slowly so that you can see the voltage rise and fall in accordance of the throttle blade opening and closing. There should not be any jumps or spikes in the voltage and it should seem like a smooth transition from closed to WOT.

Replace your throttle position sensor or the throttle body completely to resolve your P0121 OBDII DTC trouble code! Want to learn more about Honda trouble codes? Subscribe to My Pro Street today to get the latest in automotive guides, tech tips and How Tos.