Code definition P0121
Throttle pedal position sensor (TPS) / switch (TPS) Circuit Range A Performance problem.
What does code P0121 mean?
P0121 is a generic OBD-II code that indicates that the engine control module (ECM) has noticed that the learned output voltage of the TPS A sensor circuit is above or below the expected range of the sensor compared to the TPS B sensor circuit.
What causes code P0121?
- The TPS sensor circuit A of the learned range for the ECM was greater than the specified voltage difference between the TPS sensor circuit B by more than 1.6 volts.
- The learned range of the output signal from the TPS sensor circuit A to the ECM was less than the specified voltage difference between the TPS sensor circuit B and the ECM by less than 0.8 V.
What are the symptoms of the P0121 code?
- The ECM turns on the Check Engine light and goes into safe mode, cutting the throttle actuator power so that the throttle is open to within 6 degrees.
- The ECM controls fuel injection and timing to control engine speed.
- The vehicle can be driven very slowly with limited throttle response.
- The engine may run at a higher than normal idle speed and not accelerate as before.
Note: Symptoms are determined by the manufacturer’s ECM safety strategy for the vehicle during malfunction and may vary by manufacturer and vehicle.
How does a mechanic diagnose a P0121 code?
- By scanning and documenting the codes received and reviewing the freeze frame data to see when the P0121 code was set.
- Clearing the OBD-II error codes and retesting the vehicle to see if the P0121 code returns.
- Visually check all TPS connections and cables for loose or damaged components.
- Compare the data between the TPS A and B circuits on the scanner and, if incorrect, perform TPS location tests according to the manufacturer’s recommendations.
Common errors when diagnosing the P0121 code?
- Failure to follow these simple steps:
- Failure to perform a first visual inspection
- Failure to scan, document, delete, or retest codes for errors.
- Replacing the TPS sensor before testing indicates a problem.
- No comparison of TPS sensor specifications A and B before and after repair or replacement
How serious is the code P0121?
The P0121 code will cause the engine ECM to go into failure mode, causing various driving complaints until repairs are made, such as lack of acceleration, high idle speed, jerking or jolting, limited speed or stalling, depending on the manufacturer’s failure mode strategy.
What repairs can repair a P0121 code?
- Repair or replace TPS connector
- Repair wiring if necessary
- Replace the TPS with a new sensor.
Additional notes on code P0121
- The P0121 code may be accompanied by other TPS codes, so additional repair may be required for full resolution.
- Most P0121 code failures involve a faulty TPS sensor that is not tracking the throttle angle correctly.
- All TPS codes will cause the ECM to go into fault mode.
- The operational symptoms for all TPS codes will be approximately the same.
