The P0121 Code: Signs – Causes and Solutions. Car issues are an undesirable and, at times, common side effect of car ownership. Car problems have become more complex as vehicles have become more advanced. Modern automobiles are outfitted with literally hundreds of different sensors and chips to ensure proper operation. All of these sensors and chips must communicate with one another or the engine control unit (ECU) will notice a problem. When the ECU detects an issue, a diagnostic trouble code (DTC) is generated. P0121 is a typical DTC that stands for “Throttle/Pedal Position Sensor ‘A’ Circuit Range/Performance Problem.”
DTC P0121 Code Definition
The P0121: “Throttle/Pedal Position Sensor ‘A’ Circuit Range/Performance Problem” DTC indicates a fault with the engine’s Throttle Position Sensors. It is a very serious DTC that, if not addressed immediately, could cause serious engine damage. To comprehend the significance of the P0121 DTC, you must first grasp how your vehicle’s throttle body works.
Throttle Cable Systems vs. Drive by Wire Systems
The intake manifold of an internal combustion engine feeds air into the combustion chambers. The throttle body is connected to the intake manifold and receives air from the intake system or turbocharger/supercharger. A throttle blade is located within the throttle body.
The location of the throttle blade controls how much air enters the engine at any one time. The more open the throttle blade, the more air enters the engine and the harder it works, the less air enters and the engine works less hard. When driving at low speeds, the throttle blade is relatively closed, whereas when driving at high speeds, the blade is as open as mechanically possible.
This is where the distinction between drive-by-wire and throttle-cable systems becomes apparent.
The accelerator pedal position sensor (APPS) is used by the ECU in DBW systems to calculate throttle blade location. The APPS are positioned on the physical accelerator pedal and communicate to the ECU how hard the driver is pushing down. The ECU directs the throttle blade position to open more when the driver presses harder. Because the APPS communicates with the throttle blade electronically – via a wire – manufacturers refer to this as a drive by wire system.
This is not the same thing as a throttle cable system. A mechanical cable, not an APPS, connects the accelerator pedal to the throttle body in a throttle cable system. DBW systems are significantly newer and more efficient than TC systems, and most manufacturers have converted to DBWs throughout the years.
The Throttle Position Sensor (TPS) in both systems reads the position of the throttle blade and relays that information to the ECU. The ECU then adjusts the calibration to optimize fueling, ignition advance, and other parameters for maximum efficiency.
Malfunction of the Throttle Position Sensor (TPS)
As you may expect, the TPS is critical for a vehicle’s steady and safe operation. Without accurate throttle control, the car may unexpectedly bog down or jolt ahead at inappropriate times. This could result in a dangerous operation and possibly a serious accident.
If the TPS begins to emit signals that are outside of the intended range, the ECU will make improper adjustments to the throttle blade. This could result in too much or too little fuel being added at the incorrect periods. When there is insufficient gasoline, the engine will knock, splutter, stall, and fail to sustain idle. Too much fuel will result in poor gas mileage, decreased performance, and maybe stalling.
The DTC P0121 code indicates that the TPS is reading erroneously. The majority of TPS operate in the 0.5v to 4.5v range. They measured 0.5-1.0v at idle and 4.5-4.7v at full speed. If you get the P0121 code, it signifies that the TPS is sending signals to the ECU that are outside of their intended range.
P0121 Codes Related
- P0120 – Malfunction of the Throttle/Pedal Position Sensor/Switch “A” Circuit
- P0122 – Low Input Throttle/Pedal Position Sensor/Switch “A” Circuit
- P0123 – Throttle/Pedal Position Sensor/High Input Switch “A” Circuit
- P0124 – Intermittent Throttle/Pedal Position Sensor/Switch “A” Circuit Input
- P0220 – Malfunction of the Throttle/Pedal Position Sensor/Switch “B” Circuit
- P0221 – Range/Performance Issue with Throttle/Pedal Position Sensor/Switch “B” Circuit
- P0222 – Low Input Throttle/Pedal Position Sensor/Switch “B” Circuit
- P0223 – High Input Throttle/Pedal Position Sensor/Switch “B” Circuit
- P0224 – Intermittent Throttle/Pedal Position Sensor/Switch “B” Circuit Input
- P0225 – Malfunction of the Throttle/Pedal Position Sensor/Switch “C” Circuit
- P0226 – Range/Performance Issue with Throttle/Pedal Position Sensor/Switch “C” Circuit
- P0227 – Low Input Throttle/Pedal Position Sensor/Switch “C” Circuit
- P0228 – Throttle/Pedal Position Sensor/High Input Switch “C” Circuit
- P0229 – Intermittent Throttle/Pedal Position Sensor/Switch “C” Circuit Input
The fault codes for sensors “D,” “E,” and “F” continue in the same order, from P2120 to P2134, with five codes each.
- P2135 – Throttle/Pedal Position Sensor/Voltage Correlation Switch “A”/”B”
- P2136 – Throttle/Pedal Position Sensor/Voltage Correlation Switch “A”/”C”
- P2137 – Throttle/Pedal Position Sensor/Voltage Correlation Switch “B”/”C”
- P2138 – Correlation of Throttle/Pedal Position Sensor/Switch “D”/”E” Voltage
- P2139 – Throttle/Pedal Position Sensor/Voltage Correlation Switch “D”/”F”
- P2140 – Throttle/Pedal Position Sensor/Voltage Correlation Switch “E”/”F”
Meanings of Related DTC Codes
The P0121 code and drive-by-wire systems are linked via many codes. Because the DTCs read throttle/pedal, they could apply to either the TPS or APPS systems. Most vehicles have more than one TPS, and some have up to three. DTC codes “A,” “B,” and “C” relate to specific TPS. The same holds true for APPS. Some vehicles have up to three, and DTC codes “D,” “E,” and “F” refer to specific APPS.
The purpose for the TPS and APPS having up to three sensors is to provide a failsafe. If two TPS sensors do not read the same value, it indicates that one or both are faulty. This generates a DTC code ranging from P2135 to P2137. If two of the three APPS sensors do not read the same thing, a P2138 – P2140 DTC code will be shown.
Common Symptoms of the P0121 Code
There are some frequent indications that will alert you to a TPS malfunction:
- The Check Engine/Malfunction Indicator Light is illuminated.
- The engine will not start or is difficult to start.
- Sputtering, banging, or stalling of the engine while driving normally
- Idle that is rough or uneven, has an extremely high idle, or fails to maintain idle
- Engine performance is subpar.
- The vehicle has been placed in limp mode.
Because the TPS controls so much of the engine’s operation, if it fails, the symptoms will be immediately apparent in most cases. To begin, the CEL/MIL will glow and notify you of the presence of a DTC. If this occurs, you can use an OBD II code reader to check the CEL, but we’ll go over that later.
Other symptoms will be directly related to the engine’s performance. If the TPS begins to emit signals that are outside of the intended range, the ECU will make improper adjustments to the throttle blade. This could result in too much or too little fuel being added at the incorrect periods. When there is insufficient gasoline, the engine will knock, splutter, stall, and fail to sustain idle. Too much fuel will result in poor gas mileage, decreased performance, and maybe stalling.
If the car detects too much of a problem, it will immediately enter limp mode to protect itself. Limp mode drastically restricts engine performance, enabling only limited acceleration and speed.
Code Severity Level P0121
Level: Extremely High – Discontinue driving immediately.
While some DTC codes may not cause major problems, the P0121 code is not one of them. If you begin to notice the following symptoms and notice a CEL/MIL, immediately stop driving until you can determine what the code is. You’ll be pleased you did if it’s a P0121 code. Engine knocking, sputtering, and stalling could all cause major issues, with engine knocking in particular resulting in numerous blown engines.
Engine knock can be caused by insufficient fuelling, which can occur when the TPS fails. If your car anticipates a large amount of fuel, it will add an equal amount of air. When there is too much air and not enough fuel, cylinder temperatures rise, resulting in engine knock. In severe circumstances, extremely large engine knock episodes can result in catastrophic engine failure.
The vehicle will also most likely be put into limp mode, which severely alters the vehicle’s performance characteristics. It effectively removes them, and the car will only drive at slow speeds until the DTC is cleared. It is critical that you do not drive if you confirm or suspect a P0121 code is present. If you continue driving, you risk engine damage and probable accidents.
Causes of P0121 Code
There are various possible causes for the P0121 error code:
- Failure of the TPS or the TPS wiring harness
- Throttle Body Dirty/Faulty
The most obvious source of the P0121 error code is a faulty TPS. These sensors can fail and must be changed from time to time. Furthermore, the wiring harness connecting to the TPS can short, become corroded, or be damaged. Critters sneaking into the engine bay and wreaking havoc is a common cause of TPS/circuit failure. The sensor may potentially be dusty or loose, resulting in inaccurate voltage measurements.
A filthy or faulty throttle body is another possible explanation. If the throttle body becomes rusted or clogged with dirt and debris, the sensor’s readings may suffer. Similarly, if it fails, the throttle blade may operate abnormally, causing the TPS to give inaccurate readings.
Diagnosis of the P0121 Error Code: A Step-by-Step Guide
There are several methods for diagnosing the P0121 error code. Anyone who is comfortable disassembling their throttle body and using a multi meter can perform the diagnosis. If you get the P0121 code, one of three things is almost certainly wrong: a faulty TPS sensor, a broken wire harness, or a filthy throttle body.
Step 1: Using an OBD II code reader, confirm the presence of the P0121 code.
To establish which DTC is present, use an OBD II scan tool. If it reads P0121, the engine bay must be inspected.
Step 2: Examine the TPS and wiring harness visually.
First, ensure that the wiring harness is properly plugged into the TPS. If the TPS has evident damage or corrosion, replace it and reattach it. If the wiring harness is damaged, use a multimeter to determine what has to be replaced. Clear the DTC and test drive the car after repairing the harness or replacing the TPS. If the DTC arises again, the throttle body must be inspected. If the TPS and wire harness look to be in good working order, go to the next stage of checking the throttle body.
Step 3: Remove and clean the throttle body in step three.
Remove the throttle body and clean the inside using throttle body cleaning. After cleaning, replace the throttle body, clear the DTC code, and test drive. If the problem reappears, the fault is most likely with the TPS or wire harness. If you’ve already replaced the wire harness or TPS, the problem could be with the one you didn’t. However, if you have not yet touched the TPS or wiring harness, it is likely that one of them is the source of the problem.
Step 4: Check the wiring harness and TPS using a multimeter.
A multimeter is required to test the TPS and wiring harness. Check out this video for a step-by-step instruction to testing the TPS and wire harness.
If the TPS or wiring harness is malfunctioning, replace it and reset the DTC (see procedures below). The issue should now be resolved, and the DTC should no longer appear. If the DTC persists after you have verified the TPS and wire harness work and cleaned the throttle body, you will most likely need to take the vehicle to a professional for further diagnosis.
Common Mistakes in P0121 Diagnosis
When diagnosing the P0121, people frequently make the following mistakes:
- We are unable to confirm the particular DTC for P0121.
- Failure to visually inspect the TPS and wiring harness
- Failure to clean the throttle body
- Testing the TPS and wiring harness incorrectly
- Prematurely replacing the TPS
One of the most typical errors when dealing with TPS issues is failing to confirm the precise DTC. Even if you are certain that the problem is caused by a malfunctioning TPS, confirm the code first. You don’t want to waste time and money on the incorrect issue.
Make sure to thoroughly inspect the TPS and wiring harness. Something as simple as a disconnected or loose wiring harness or sensor could be solved quickly and easily. It’s also critical to clean the throttle body. A clogged throttle body might cause sensor issues. Even if cleaning the throttle body does not resolve the problem, it may prevent it from occurring in the future.
It is also critical to test the TPS and wiring harness correctly. Incorrect readings may result in failing to diagnose the actual problem and moving on too hastily. It may also result in unneeded replacements. Only take these steps if you are confident in your ability to complete them correctly.
Finally, replacing the TPS before confirming that it is the source of the DTC is a bad idea. While it is usually the source of the problem, attempting to repair something that is not broken is a waste of time and money.
P0121 Code Solution and Correction
If you’ve determined that the problem is caused by the TPS, the solution is simple.
- Disconnect the battery and turn off the vehicle.
- Remove the hood and look for the TPS sensor and throttle body.
- Unplug the TPS sensor’s electrical harness.
- Remove the throttle body’s TPS.
- Screw the new TPS into place on the throttle body.
- Reconnect the electrical harness to the new TPS.
- Clear the DTC and put the car through its paces.
The P0121 error number should not reoccur after successfully installing the updated TPS.
P0121 Costs of Diagnosis and Repair
The P0121 can be diagnosed and repaired for a low cost. The only costs if you do everything yourself are a multimeter and a new TPS. The TPS costs from $30 to $150, depending on the brand/manufacturer. If possible, we recommend using an OEM part. If you take it to a shop, the labor fees will be determined by the shop, which will most likely be 1-2 hours. Labor is normally priced between $120 and $150 per hour, plus the cost of the TPS.
P0121 Code Synopsis
The P0121 DTC is not the most prevalent, but it appears to affect a significant number of vehicles. GM/Chevrolet appear to be the worst offenders, but TPS is used in practically every car. You now have the information and resources to diagnose and repair the P0121 code if it occurs in your car.