The OBDII trouble code P1121 is due to your throttle position sensor signal being lower than expected by your ECU. Problems that arise from this OBDII code can range from annoying symptoms like sluggish driving, bad throttle response, poor fuel economy to dangerous.
Today we’ll be testing this OBDII trouble code in our 1996 Honda Civic EX, and showing you how to resolve the P1121 OBDII trouble code. You will also know how to service your Civic throttle position sensor, as well as test and re-install.
The throttle position sensor is located opposite the throttle body valve on your D16Y8 and is connected via a three pin weatherproof connector. Because your OBDII error has to do with the signal being received by your ECU, the wire we’ll be referencing here is specifically the signal.
Unfortunately, the TPS is underneath all of your intake ducting and will need you to reach back there to test each of the three wires individually.
To access your throttle position sensor, you will need to remove all your factory air ducting and panels. You will need a flathead screwdriver, a 8mm socket and 10mm socket. To begin our how to service your Civic throttle position sensor, unplug your Air Intake Temperature (AIT) sensor.
After unplugging your air intake temperature sensor, inspect the terminal to make sure it’s free of any debris. Now unclip your factory airbox and snorkel. You do not need to disconnect the intake snorkel here, just unclip and push the entire thing to the side.
Now you should be ready to pull off your factory air filter, leaving you with the throttle body and the base of your intake box.
Take off the stock air filter element, if you have a K&N or HKS unit to replace it now is the time to get that ready.
Next disconnect the air tube by undoing the tension in this alligator clip and sliding the metal clip downward. Pull off the vacuum hose carefully and leave this disconnected.
Now take your 8mm socket and undo this 8mm mount bolt that secures the bottom of your air box. This is located on the left hand side of your intake manifold or the passenger side.
With this mount bolt removed, reach down around your throttle body inlet and locate the 8mm bolt that holds the clamp tension. Loosen this 8mm bolt and you should be able to carefully wiggle the intake box base off the throttle body.
Remember that the throttle position sensor is located directly across from the throttle butterfly blade. The TPS sensor is a 3 wire sensor and will need to be tested with the sensor connected.
The middle wire is the one you’ll need to tap to test the voltage range being sent to the ECU. Use a voltmeter or a similar device that can read voltage on the fly to test the middle pin now. Make sure the ignition is turned to the “ON” position but the motor is not running.
Have a friend sit in the car and press on the throttle or turn the throttle blade with your other hand. The theory here is that you will be testing the voltage signal being sent to your ECU during the throttle body sensor’s full range. You should see the voltage range at .4 to .9 with the throttle plate completely closed, and 4.5 volts at wide open throttle.
This range should be smooth and linear if your multimeter is fast enough, and without any huge chunks of voltage jumps. Try to vary the speeds in which you open and close the throttle body to see this action slower. Make sure there are no ‘jumps’ in the voltage range that seem disproportionate to the amount you are turning the throttle blade.
If you do not see this range of voltage while the throttle plate is being open and closed, then check the yellow wire for voltage. There should be voltage at the throttle position sensor while connected.
If you see a full range range then this means you throttle position sensor is working correctly, and that your problem may lie elsewhere. Try resetting your ECU and clearing your codes to see if the P1121 trouble code returns. It may be an intermittent issue that can take time to resurface.
CHECKING AT THE ECU
Don’t want to crawl back behind your hot motor and check your throttle position sensor? You can simply check it at the ECU connector directly in our 1996 Honda Civic EX VTEC. Check your passenger side floorboard, which will have 2 plastic push pins that hold in the panel.
Remove this panel to access the P2E Honda ECU and wiring harness.
Undo the 10mm bolts that hold the ECU in place, and pull gently towards you. Flip the ECU towards you so that you can see the wiring harness, and the plug you are wanting is the D plug located at the end of the ECU.
The pin you are looking to test is the throttle position sensor wire, which is D1 or the first pin on the D plug, circled above. Test this pin for the throttle position sensor range, making sure to check for voltage range of .4 to 4.5 volts. We’ve unplugged the D plug to illustrate which pin you are looking for.
If you are still confused, check our ECU PINOUT section for further details.
That does it for our how to service your Civic throttle position sensor, please leave us any comments or questions below!