P0121 Code – Symptoms, Causes, and Fix

ByChandler Published on Updated on

Car problems are an unfortunate and at times frequent symptom of car ownership. As cars have become more complex, car problems have become more complex, too. Modern cars are equipped with literally hundreds of different sensors and chips to make sure they run correctly. All of these sensors and chips have to work together or else the engine control unit (ECU) detects a problem. When the ECU detects a problem it throws a diagnostic trouble code (DTC). A common DTCs is the P0121 “Throttle/Pedal Position Sensor ‘A’ Circuit Range/Performance Problem.”

P0121 Code Check Engine Light illuminated
Credit: Wikiuser100000/Wikipedia

Table of Contents

DTC P0121 Code Meaning

The P0121: “Throttle/Pedal Position Sensor ‘A’ Circuit Range/Performance Problem” DTC indicates a problem with how the Throttle Position Sensors in the engine are operating. It is a very serious DTC and could lead to serious engine damage if it’s not taken care of quickly. To understand the importance of the P0121 DTC, it’s crucial to know how the throttle body on your car operates.

Drive by Wire vs Throttle Cable Systems

In an internal combustion engine, air is fed into the combustion chambers from the intake manifold. Connected to the intake manifold is the throttle body, which takes in air from the intake system or turbocharger/supercharger. Within the throttle body is what is known as a throttle blade.

The position of the throttle blade determines how much air enters into the engine at a time. The more open the throttle blade the more air enters the engine and the harder it works, the more closed it is the less air enters and the engine doesn’t work as hard. At low speed driving, the throttle blade is relatively closed, while at wide open throttle the blade is as open as mechanically possible.

This is where the difference between drive-by-wire and throttle-cable systems emerges.

In DBW systems, the ECU determines throttle blade positioning based on the accelerator pedal position sensor (APPS). The APPS are located on the physical accelerator pedal and relay how hard down the driver is pushing to the ECU. The harder the driver pushes down the more the ECU tells the throttle blade position to open. Manufacturers call this a drive by wire system, because the APPS communicates to the throttle blade electronically – through a wire.

This is different from what is known as a throttle cable system. In a throttle cable system, a mechanical cable links the accelerator pedal to the throttle body, not APPS. DBW systems are much newer and more efficient than TCs, and most manufacturers have shifted to DBWs over the years.

In both systems, the Throttle Position Sensor (TPS) works to read the position of the throttle blade and relay that information back to the ECU. The ECU then makes calibration adjustments to optimize fueling, ignition advance, and other parameters to achieve the most efficiency.

Throttle Position Sensor (TPS)  Malfunction

As you might imagine, the TPS is incredibly important for steady and safe operation of a vehicle. Without precise operator control over the throttle, the car could unexpectedly bog down or jerk forward at inopportune moments. This could lead to very unsafe operation and potentially to a serious accident.

If the TPS starts giving off signals outside of the predetermined range, that means the ECU will make incorrect adjustments to the throttle blade. This could inadvertently lead to too much or too little fuel being added at the wrong times. Too little fuel will result in knocking, sputtering, stalling, and failing to maintain idle. Too much fuel will result in poor gas mileage, diminished performance, and potentially in stalling issues.

The DTC P0121 code means there is a problem with the TPS reading incorrectly. Most TPS operate in the 0.5v to 4.5v range. They read 0.5-1.0v at idle and 4.5-4.7v at wide open throttle. If you get the P0121 code, it means TPS is reporting signals outside of the intended range to the ECU.

  • P0120 – Throttle/Pedal Position Sensor/Switch “A” Circuit Malfunction
  • P0122 – Throttle/Pedal Position Sensor/Switch “A” Circuit Low Input
  • P0123 – Throttle/Pedal Position Sensor/Switch “A” Circuit High Input
  • P0124 – Throttle/Pedal Position Sensor/Switch “A” Circuit Intermittent Input
  • P0220 – Throttle/Pedal Position Sensor/Switch “B” Circuit Malfunction
  • P0221 – Throttle/Pedal Position Sensor/Switch “B” Circuit Range/Performance Problem
  • P0222 – Throttle/Pedal Position Sensor/Switch “B” Circuit Low Input
  • P0223 – Throttle/Pedal Position Sensor/Switch “B” Circuit High Input
  • P0224 – Throttle/Pedal Position Sensor/Switch “B” Circuit Intermittent Input
  • P0225 – Throttle/Pedal Position Sensor/Switch “C” Circuit Malfunction
  • P0226 – Throttle/Pedal Position Sensor/Switch “C” Circuit Range/Performance Problem
  • P0227 – Throttle/Pedal Position Sensor/Switch “C” Circuit Low Input
  • P0228 – Throttle/Pedal Position Sensor/Switch “C” Circuit High Input
  • P0229 – Throttle/Pedal Position Sensor/Switch “C” Circuit Intermittent Input
  • The fault codes continue from P2120 – P2134 for sensors “D,” “E,” and “F” in the same order, five codes each.
  • P2135 – Throttle/Pedal Position Sensor/Switch “A”/”B” Voltage Correlation
  • P2136 – Throttle/Pedal Position Sensor/Switch “A”/”C” Voltage Correlation
  • P2137 – Throttle/Pedal Position Sensor/Switch “B”/”C” Voltage Correlation
  • P2138 – Throttle/Pedal Position Sensor/Switch “D”/”E” Voltage Correlation
  • P2139 – Throttle/Pedal Position Sensor/Switch “D”/”F” Voltage Correlation
  • P2140 – Throttle/Pedal Position Sensor/Switch “E”/”F” Voltage Correlation

There are several codes related to the P0121 code and drive by wire systems. You’ll notice the DTCs read throttle/pedal, which means they could apply to either the TPS or APPS systems. Most cars have more than one TPS, and some cars have as many as three. DTC codes for “A,” “B,” and “C,” refer to individual TPS. The same thing applies to the APPS. Some cars have as many as three, and DTC codes “D,” “E,” and “F” refer to individual APPS.

The reason for both the TPS and APPS having as many as three sensors is to be a failsafe. If two of the TPS sensors are not reading the same it indicates one or both are malfunctioning. This throws a DTC code from P2135 – P2137. If two of the three APPS sensors are not reading the same it indicates the same thing, and a P2138 – P2140 DTC code will come up.

P0121 Code Common Symptoms

There are some common symptoms that will give you an indication of a TPS malfunction:

  • Check Engine Light/Malfunction Indicator Light is on
  • Engine won’t start/is hard to start
  • Engine sputtering, knocking, or stalling during normal driving
  • Rough or uneven idle, abnormally high idle, or failure to maintain idle
  • Poor engine performance
  • Car is thrown in limp mode

Since the TPS controls so much related to the operation of the engine, if it starts to go out the symptoms will be immediately noticeable in most cases. For starters, the CEL/MIL will illuminate and alert you to the presence of a DTC. If this happens, you can check the CEL with an OBD II code reader, but we’ll get into that later.

Other symptoms are going to be directly related to the performance of the engine. If the TPS starts giving off signals outside of the predetermined range, that means the ECU will make incorrect adjustments to the throttle blade. This could inadvertently lead to too much or too little fuel being added at the wrong times. Too little fuel will result in knocking, sputtering, stalling, and failing to maintain idle. Too much fuel will result in poor gas mileage, diminished performance, and potentially in stalling issues.

If the car senses too much of an issue, it will automatically be thrown into limp mode to protect itself. Limp mode severely limits the engine’s performance, allowing for only partial acceleration and speed.

P0121 Code Severity Level

Level: Very High – Stop driving the car immediately

While some DTC codes might not present serious issues, the P0121 code is not one of those. If you start to experience the above symptoms and see a CEL/MIL, immediately stop driving the car until you can find out what the code is. If it’s a P0121 code you’ll likely be glad you did. Engine knocking, sputtering, and stalling could lead to big problems, with engine knocking in particular leading to many blown engines.

Engine knock can be caused by insufficient fueling, which might happen with malfunctioning TPS. If your car is expecting there to be a lot of fuel it adds a corresponding amount of air. If there is too much air and not enough fuel, cylinders temperatures spike which leads to engine knock. Really big engine knock events can lead to catastrophic engine failure in severe cases.

The car will also likely be thrown into limp mode, which drastically changes the car’s performance characteristics. It basically removes them, and the car will only drive at limited speeds until the DTC is cleared. It’s very important that you do not continue to drive if you confirm/suspect a P0121 code is present. You risk engine damage and potential accidents if you continue driving.

P0121 Code Causes

There are several potential causes of the P0121 code:

  • Defective TPS or TPS wiring harness failure
  • Dirty/Faulty Throttle Body

The most obvious cause of the P0121 code is going to be a defective TPS. These sensors can go bad from time to time and need to be replaced. In addition, the wiring harness connecting to the TPS can get a short, or become corroded or damaged. One common cause of TPS/circuit failure is from critters getting into the engine bay and wrecking havoc. The sensor could also be dirty or loose, causing incorrect voltage readings.

Another potential cause is a dirty or defective throttle body. If the throttle body becomes corroded or full of dirt and junk it could easily affect the sensor’s readings. Likewise, if it is failing the throttle blade could behave irregularly and cause the TPS to give incorrect readings.

Step-by-Step Guide to Diagnosing the P0121 Code

There are a few ways to diagnose the P0121 code to figure out the problem. The diagnosis can be done by anyone comfortable with taking apart their throttle body and also using a multi meter. If you get the P0121 code it is almost guaranteed to be one of the three things: a faulty TPS sensor, a faulty wiring harness, or a dirty throttle body.

Step 1: Verify the P0121 code is present through an OBD II code reader.

Use an OBD II scan tool to determine which DTC is present. If it reads P0121 the next step is to inspect the engine bay.

Step 2: Visually inspect the TPS and wiring harness

First, make sure the wiring harness is correctly plugged into the TPS. If there is any obvious damage or corrosion to the TPS, replace the TPS and reinstall. If the wiring harness has damage, test it with a multi meter to determine what needs replacement. After fixing the harness or replacing the TPS, clear the DTC and test drive the vehicle. If the DTC reappears the next step is to inspect the throttle body. If the TPS and wiring harness appear fine, move on to the next step of inspecting the throttle body.

Step 3: Remove and clean the throttle body. 

Remove the throttle body and use throttle body cleaner to clean the inside. After cleaning, reinstall the throttle body and clear the DTC code and take a test drive. If the problem reappears then the issue is with the TPS or wiring harness. If you have already replaced either the wiring harness or TPS, the issue could be with the one you did not replace. However, if you did not touch the TPS or wiring harness yet it is likely one of them is the problem.

Step 4: Using a multi meter, test the wiring harness and TPS

To test the TPS and wiring harness you will need a multimeter. For a step by step guide to testing the TPS and wiring harness check out this video.

YouTube video

If the TPS or wiring harness is faulty then replace the damaged one and reset the DTC (instructions below). The problem should now be solved and the DTC should not reappear. If you still have the DTC after verifying the TPS and wiring harness work and cleaning the throttle body, you will likely have to take the car into a professional for them to diagnose the issue further.

Also Read: Everything about Fault Code P0141

Common Mistakes When Diagnosing the P0121

There are a few common mistakes people often make when diagnosing the P0121:

  • Not confirming the specific DTC for P0121
  • Not visually inspecting the TPS and wiring harness
  • Not cleaning the throttle body
  • Incorrectly testing the TPS and wiring harness
  • Replacing the TPS prematurely

One of the most common mistakes when addressing issues with the TPS is not confirming the specific DTC. Even if you are convinced that the issue is related to a faulty TPS, make sure you confirm the code first. You don’t want to spend time and money fixing the wrong problem.

Make sure you give the TPS and wiring harness a thorough look over. Something easy like an unplugged or loose wiring harness or sensor could be very easy and quick to solve. It’s also important to actually clean the throttle body. A dirty throttle body can lead to problems with the sensor. Even if cleaning the throttle body does not fix the issue, it could prevent it from happening in the future.

Correctly testing the TPS and wiring harness is also of paramount importance. Incorrect readings could lead to not diagnosing the correct issue and moving on too quickly. It could also lead to unnecessary replacements. Only do these steps yourself if you are comfortable and capable of doing them properly.

Finally, prematurely replacing the TPS before verifying it is the cause of the DTC would be a mistake. While it is usually the problem, it’s a waste of time and money to fix something that is not broken.

P0121 Code Solution and Fix

If you have verified that the issue is related to the TPS, the fix is relatively easy.

  • Step 1: Turn off the car and disconnect the battery
  • Step 2: Open the hood and locate the TPS sensor and the throttle body.
  • Step 3: Unplug the wiring harness from the TPS sensor.
  • Step 4: Unscrew the TPS from the throttle body.
  • Step 5: Screw-in the new TPS to the throttle body.
  • Step 6: Plug the wiring harness back into the new TPS.
  • Step 7: Clear the DTC and test drive the vehicle.

After successfully installing the new TPS the P0121 code should not reappear.

P0121 Diagnosis and Repair Costs

Diagnosing and repairing the P0121 is relatively inexpensive. If you do everything yourself, the only expenses are a multi meter and new TPS. The TPS runs from $30 – $150, depending on brand/manufacturer. We recommend going with an OEM part if possible. If you take it to a shop they will determine their own labor costs, which will likely be 1-2 hours. Labor is typically around $120 – $150 an hour, plus the cost of the TPS.

P0121 Code Summary

The P0121 code is not the most common DTC to appear, but it does seem to affect a decent amount of vehicles. GM/Chevrolet seem to be some of the most prone offenders, but nearly every car utilizes TPS. Now, you have the knowledge and tools to diagnose and fix the P0121 code should it happen to your vehicle.

Do you have any experience with the P0121 code with your vehicle? Was it a quick and easy fix or did you need to take it to a professional? Let us know in the comments below!

Leave a Reply