Sensors are everywhere in modern vehicles. Which is great, until one of them fails. At that point, you’ll most likely get a check engine light code on your dash. The P0121 is one of the many sensor-related trouble codes you might see as a car owner.  

In this particular case, the code is related to the throttle position sensor, but it doesn’t necessarily mean that this part has gone bad. To help you figure out what’s going on with your car, we’ll give you a detailed rundown of this code, what it means, and how you can go about fixing it. You’ll need some basic tools as well as an OBD II scanner to read the codes. With that in hand, you’re ready to tackle this issue and potentially fix your car in your own garage! 

What is the P0121 Code, and What Does It Mean?

The full definition of the P0121 code is “Throttle Position Sensor Circuit ‘A’ range/performance.” This code appears when the engine computer detects that your throttle position sensor (TPS) is operating outside its normal range, specifically circuit A. The ECU uses either the MAP sensor data or a set reference voltage to track the performance of the TPS. Once it notices a discrepancy in these data sets, you’ll get a check engine light (CEL) with this code. 

On top of that, your vehicle will enter limp mode as throttle issues can be a safety hazard.

What is a Throttle Position Sensor?

A throttle position sensor measures how far the throttle valve is open and sends that information to the PCM. The PCM uses this information to adjust ignition timing and fuel injection together with info from other sensors in the vehicle.

The throttle position sensor uses two circuits (A and B). This code means that circuit A is operating out of range.

What Causes the P0121 Code?

When you look at engine-related trouble codes, you’ll find that many of them share the same or similar causes. This also applies to the P0121 code. There are some unique issues that can cause this trouble in code, but you still need to assess the bigger picture here. In the end, you get a list that looks something like this:

• A faulty throttle position sensor 
• Damage to the wiring harness
• Corroded sensor connector
• Symmetrical sensor failure

Faulty Throttle Position Sensor 

This code is related to the ECU receiving mismatched throttle position sensor data. Failure of the sensor itself is common when you have a coolant leak or in a vehicle with an accident history.

A common point of failure is corrosion on the sensor connector or internal damage caused by excess heat. Throttle position sensors are meant to survive the life of the vehicle. As a result, they usually only fail due to hard-to-predict factors such as kinetic damage resulting from a car accident, or similar.

Damage to the Wiring Harness

This is by far the most common reason you have a P0121 trouble code. As the harness is in the engine bay, it is exposed to repeating high/low-temperature cycles. The constant temperature changes between hot and cold can make the wiring insulation brittle, ultimately leading to a break in the circuit or a short somewhere. 

Symmetrical Sensor Failure

There are other sensors that work together with the TPS to feed similar information to the ECU. These are the manifold absolute pressure sensor (MAP), mass air flow sensor (MAF), and accelerator pedal position (APP) sensors.

They can experience similar issues to the throttle position sensor, such as wiring damage and internal/external corrosion. When they fail, they cause the TPS numbers to conflict with the engine computer reference numbers. As a result, you might get a P0121 code.

What Are the Common Symptoms of P0121 Fault Code?

Identifying the symptoms of an issue can often tell you a lot about what’s going on with your car. Especially when there could be more than one thing causing that issue. The P0121 code shows a couple of common symptoms that include:

• Check engine light
• Inhibited throttle response
• Limp mode
• Rough idle

Check Engine Light

The check engine light is your way of instantly knowing that something is wrong with your vehicle. As this particular code is considered severe, the light will flash on top of just illuminating. That should be your cue to pull over. 

Inhibited Throttle Response

This is a common symptom when you have an issue with the TPS. It comes from the ECU misinterpreting how much the throttle body is open. Thus, it applies more fuel or adjusts ignition timing based on this. Jerky acceleration and slow or no response to pedal input are some signs of this issue.

Limp Mode

Unintended acceleration is a major safety hazard. To prevent this, most manufacturers implement a mode where the ECU limits the throttle, or the gear changes if your car runs an automatic transmission. It doesn’t tell you much about the P0121 code, but a limp mode is often a clear indicator that something serious is wrong with your vehicle. 

Rough Idle

The nature of the data the throttle position sensor sends means that when it fails, you often end up with a lean or rich running condition. In the event either of these conditions occurs, they will affect your engine’s idle and cold start behavior. Although this is a common symptom of other issues, it also applies to the P0121 code.

How to Diagnose and Fix the Cause of P0121 Code 

The throttle position sensor’s location makes it easy to diagnose and fix. However, finding what exactly failed is a bit more tricky. As with most other engine trouble codes, you’ll want to start with a full scan of the car. 

OBD II Reader

Using a scan tool allows you to identify the P0121 code and any additional codes. Connect your OBD II reader to the vehicle via the connection port that is usually located somewhere under the dash on the passenger side. 

You’ll want to run a complete scan of the system. That way you’ll get a complete picture of what’s going on with the car. The P0121 may not be the only code you find in there. If you have more than one code, there is a chance that your diagnostic process takes you in an unforeseen direction. 

Live data is also a helpful tool in diagnosing this issue. You can check how far the throttle valve is opening and whether the MAP, MAF, and app sensors are functioning.

Visual Inspection

This is a crucial step in diagnosing a sensor issue. In general, these components fail in a way that often makes the damage visible.

Look for any signs of corrosion, melting, or mechanical damage to the TPS sensor, related sensors or their respective wiring harnesses. Give the connectors a bit of a pull to make sure they are connected in the correct fashion.

Throttle Position Sensor Test

Because there are many types of throttle position sensors, you should always consult your owner’s manual when testing. That being said, the testing method and numbers you should read are similar.

Now that we’ve made sure there is no external damage, we can test for internal wire breaks. You need a multimeter or oscilloscope to test for voltage. The red probe should contact the wire and the black probe with the ground. The ground is a part of the vehicle chassis.

At this stage, your vehicle needs to be in the KO/EO (Key-On Engine Off). The TPS signal wire reading should be around 5V. Your power and your ground wire’s normal operating voltage is approximately 12V.

Take note that there is an additional code for the low voltage of the TPS sensor. This code is P0122. To test whether the throttle is functioning correctly, we need to disconnect the air box, so we have access to the throttle body.

Start your vehicle. When the throttle is fully closed and there is no pedal pressure, the voltage should be 0V – 2V. Upon full pedal depression, your voltage should be 5V.

Comparing Circuit “A” and “B”

The P0121 is related to circuit A’s numbers being abnormal. Because most throttle position sensors use two circuits, you may not have the trouble code for circuit B. In this scenario, you can compare the two numbers. If they are not the same, it means that the root of the issue is not fixed.

Testing Related Sensors

Of course, other sensors have their own trouble codes and testing guides. It is vital that you test these sensors as thoroughly as the TPS. Mainly, because they are just as easy to test and diagnose and because they can cause the P0121 code.

How Do You Fix a Low Voltage Throttle Position Sensor?

Carbon buildup is a common cause of throttle issues. Luckily, it’s also fairly simple to fix if it is not too far gone. A good carbon cleaner will help loosen hardened soot. You can wipe it away with a brillo pad or rag.

A mechanical throttle position sensor can be adjusted manually. However, this leaves it vulnerable to mechanical failure. You can adjust it using a screwdriver and run through the throttle position sensor test again.

If your numbers are still abnormal, it’s likely that the sensor needs to be replaced. If you aren’t comfortable adjusting the sensor manually, many mechanics will do it free of charge.

Does P0121 Clear Itself?

This depends on whether your vehicle uses mechanical or electronically controlled throttle. Many manufacturers will not allow you to disengage the limp mode until the error is cleared.

Regardless, you can clear the code using an OBD-II reader. In the event you can’t access one, as long as the fixed root cause of the issue, it should be safe to drive.

Is It OK to Drive with P0121?

While you technically can drive your car with a P0121 code present, you should only do so to get your car home. Throttle issues are not to be taken lightly, so do your best to address any such problems as soon as you can. 

Fix the P0121 Code Using Quality Parts 

Now that you know how to diagnose and fix the P0121 code, you need the parts for the job. Our parts store, eEuroparts.com, has all the components you might need, and more. To find parts that are a guaranteed fit, all you need to do is pick your vehicle from the dropdown list. We’ll take care of the rest!