P0121: TP Sensor A Circuit Range Performance Problem

What Is P0121?

P0121 is a generic OBD-II Diagnostic Trouble Code (DTC) defined as “Throttle Position Sensor (TPS) “A” Circuit Range Performance Problem.”

Your vehicle’s Powertrain Control Module (PCM) or Engine Control Module (ECM) registers this circuit range performance problem and activates the check engine light when it detects that the Throttle Position Sensor switch A doesn’t send an appropriate TPS signal.

The throttle position sensor (TPS sensor) is a potentiometer located on the throttle body assembly to measure the throttle blade’s opening. 

Several old vehicles with a mechanical throttle body may have just one TPS sensor, while new makes and models come with two sensors (TPS “A” and “B”)  attached to the throttle body assembly. 

The TPS sensors send a reference voltage to your car’s PCM, depending on your vehicle’s throttle position. At the idle position, the TPS voltage is 0.5 volts, and at Wide Open Throttle (WOT), the TPS voltage is 4.5 volts. 

When your vehicle registers code P0121, the throttle position sensor “A” sends a reference voltage above or below its expected range. Some vehicles may also set code P0121 when the TPS signal doesn’t match up with the Manifold Absolute Pressure Sensor (MAP sensor) signals. 

Note: Vehicles with Electronic Throttle Control may use a pair of accelerator pedal position sensors attached to the gas pedal to control the throttle plate. In that case, the code P0121 may point to a problem with the accelerator pedal position sensor instead of the TPS sensor.

Common Symptoms

Besides having an illuminated check engine light, here are the other symptoms associated with code P0121: 

• Vehicle knocking or jerking 
• The engine is hard to start 
• Engine hesitates during acceleration
• Engine stalling 
• Limited throttle response 
• Rough idling 
• The vehicle goes into the failsafe mode or limp mode
• Reduced top engine speed

Note: In failsafe mode, the Powertrain Control or Engine Control Module cuts the current to the throttle actuator and puts the throttle valve at less than 6O opening. Less current to the throttle actuator allows the ECU or the powertrain control to limit your engine speed and restrict gear transmission to reduce the risk of car system failure.              

That said, the exact symptoms associated with the P0121 code may vary depending on your car’s engine control strategy for handling a system failure.

Can I Still Drive With a DTC P0121?

A faulty throttle position sensor is a serious check engine code, as it will cause your vehicle to go into failsafe mode. As a result, you may face several drivability issues like limited throttle response, rough idling, knocking, jerking, and stalling due to limp mode.

If you can’t find the root cause of the P0121 code, you can develop more severe and possibly costly vehicle problems. 

So if your check engine light turns on due to the P0121 code, it’s best to stop driving your vehicle immediately and get the code diagnosed and fixed ASAP.

P0121 Trouble Code Causes

Several issues could trigger your engine light for the P0121 trouble code, including a mechanical problem with the throttle body assembly or a bad TPS connector.

Here are the more common ones:  

• A faulty throttle position sensor (throttle pedal position sensorswitch)
• A defective or dirty throttle body
• An open or short TPS circuit
• Poor connection of the TPS connector
• Corrosion on the TP sensor connector pins
• A faulty pedal position sensor on the gas pedal
• Fault in the pedal position sensor circuit 
• Fluctuating MAP sensor voltage

Note: If your powertrain control module registers other codes along with DTC P0121, the causes may differ from those listed above.

P0121 Diagnosis

Code P0121 can be triggered on any vehicle, regardless of whether it’s a Volkswagen AG or a Toyota Motor Corporation model. It could be due to several reasons, from a dirty or faulty throttle body, a damaged connector, to even a faulty MAP sensor. 

Here’s how a mechanic will diagnose the root cause of this trouble code: 

1. They’ll use an OBD-II scan tool to ensure no other trouble code is present. If they find other codes, like the P0101 or  P0301, they’ll address them first.

2. Your mechanic will clear the code and test drive your vehicle to check if the code returns.

3. They’ll then use the scan tool to read the idle and wide-open throttle readings and compare them with your vehicle’s manual specifications. If the readings are off, it indicates an issue with the throttle position sensor, wiring, or connector.

4. They’ll visually inspect the sensor wiring and connector to check for any signs of corrosion. They’ll also check the wiring harness for any fraying or breakage, then repair any issues to the wiring harness.

5. Next, they’ll remove the intake pipe from the throttle body to clean it. They’ll spray some throttle body cleaner on a rag and wipe all the carbon off the throttle plate and the body.

6. If the code persists, they’ll use a multimeter or a scan tool to check the minimum and maximum voltage signal to identify any open or short TPS circuit A.

Possible Repairs & Costs for a P0121

While P0121 is a generic powertrain fault code, the specific repair steps can vary depending on your vehicle’s make and model. So things might differ between a Toyota Motor Corporation vehicle and an Audi, for instance. 

Typically, the repairs begin with an hour of diagnosis time, costing between $75-$150.

Once your mechanic diagnoses the root cause, they may suggest the following repairs:

• Wiring repair/replacement: $100-$1000
• Throttle body cleaning: $220-$290
• Throttle position sensor replacement: $170-$230
• Throttle body replacement: $580-$690

Note: The above estimates include the part cost and labor charges. 

If other trouble codes accompany code P0121, a P0121 fix would need to be done along with additional repairs to ensure the code doesn’t reappear.