DTC P0121 – How To Test a Corolla TPS

DTC P0121 – How To Test a Corolla TPS


Problems with your Toyota Corolla can arise when your throttle position sensor (TPS) has failed, or is having communication problems. Your Corolla TPS can cause your Toyota to buck or idle inconsistently, with very poor throttle response. Your Toyota ECU requires this input to accurately control and orchestrate the 1.8 liter 1ZZFE fuel and spark events.

The part number for this three pin throttle position sensor from Toyota is 89452-02020, 89452-10040 and 89452-20130. If your local Toyota dealership doesn’t have this part in stock, you can also interchange for Chevy or GM part numbers of 94856828 or 94859012.


Today I’ll be showing you how to test a Corolla TPS using a multimeter to test the TPS circuit. You will be testing to make sure that your throttle position sensor is powered and sending the signal back to your 1ZZFE ECU correctly.


In order to learn how to test a Corolla TPS, you will need to open your Toyota hood and locate your Throttle Position Sensor. This sensor is mounted directly opposite the throttle blade, and is basically a big potentiometer that informs your ECU how open or closed your throttle blade is.


When your ECU receives a signal that’s outside of the normal operating range for this throttle position sensor (0-5 volts), the P0121 trouble code will be triggered. This means that the throttle position sensor may be working correctly, but may require adjustment.


Once you have the throttle position sensor connector unplugged, refer to the chart below for the Toyota Corolla 1ZZFE throttle position sensor wiring diagram.


We’ll begin our how to test a Corolla TPS DIY Guide by testing the signal wire on your Throttle Position Sensor. We’ll round out our guide by testing for power and ground as well at the engine harness side of the TPS connector.

How To Measure Toyota Corolla throttle position sensor signal


In this segment, I’ll be showing you how to measure the signal being sent from your 1ZZFE throttle position sensor. Backpin PIN C on the TPS in your Corolla and using a multimeter we’ll be measuring the output signal wire on the three wire TPS connector. Connect your TPS back together with your backpin or paper clip sticking up so that you can measure the voltage being transmitted.

If you are not going to disconnect the TPS for this test, no problem. Simply backpin the light green wire (PIN C) and proceed to measuring the voltage range, as this is what the problem is according to this P0121 trouble code.

Put the red post of your multimeter on the backpinned C PIN, and ground the black lead on the negative terminal of your battery and this should give you a voltage. This is the signal wire from your Corolla TPS.

It should measure close to 0 volts with the throttle closed, between the range of .35-.55 of voltage at closed position. You can either manually crank open the throttle, or have a friend sit in the car and depress the gas pedal all the way down.

At wide open throttle, you should see close to 5 volts worth of TPS signal. If your findings are within this range, your P0121 OBDII trouble code may be intermittent, which is the worst kind. Clear the code using a scan tool and see if it returns.

If you do not have this operating range at PIN C of your Corolla TPS, then your TPS either needs adjustment or to be replaced all together. After replacing the TPS, you should be able to clear the check engine trouble code to fix your P0121 check engine light.

How To Measure the Corolla TPS for power and ground


Now ground the black lead of your multimeter, and unplug your Corolla TPS. Measure PIN A of the ENGINE side of the TPS connector using the red probe of your multimeter. Never force or push the multimeter lead into plastic connectors, as further damage could occur.

You should have 5 volts of power at this pin with the key turned to the “ON” position. This is a yellow wire in the position of PIN A. If you do have power at this stage, go ahead and measure the TPS for the ground signal, which is PIN B. This wire is brown, and should be the ground signal for your Toyota Corolla TPS.

Once you have your TPS replaced or adjusted, you can clear the P0121 OBDII trouble code using a scan tool and test drive your Toyota. If the code does not return, you have successfully learned how to test a Corolla TPS and fix your P0121 check engine light. Have any questions on this DIY guide? leave them for us below!


    • Hi Larryboy, thanks for reading.

      What year is your Corolla? Depending on what year it is you may or may not be able to remove your TPS independent of the throttle body, but it’s not a fun job in a 1ZZFE setup. Let us know what year and we can assist you further.