Error P0121 – how to fix?

Code Definition P0121

Throttle Pedal Position (TPS) Sensor / Switch (TPS) Circuit Range A Performance problem

What does the code P0121 mean?

P0121 is a generic OBD-II code which indicates that the engine control module (ECM) has noticed that the learned output voltage of TPS sensor circuit A is above or below the expected range of the sensor compared to circuit B of the TPS sensor.

What causes the code P0121?

• TPS sensor circuit A learned range to the ECM was greater than the specified voltage difference between TPS sensor circuit B by more than 1.6 volts.
• The learned range of the output from TPS sensor circuit A to the ECM was less than the specified voltage difference between TPS sensor circuit B and ECM by less than 0.8 volts.

What are the symptoms of the P0121 code?

• The ECM turns on the Check Engine light and enters safety mode, cutting power to the throttle actuator to bring the throttle open to approximately less than 6 degrees.
• The ECM controls the fuel injection and the timing to control the engine speed.
• The vehicle may be driven very slowly with limited throttle response.
• The engine may idle higher than normal and not accelerate as before.

Note: Symptoms are determined by the manufacturer’s ECM safety strategy for the vehicle during a fault and may vary by manufacturer and vehicle.

How does the mechanic diagnose the P0121 code?

• By scanning and documenting the received codes and reviewing the data from the frozen frames to see when the P0121 code was set.
• Clearing OBD-II error codes and re-testing the vehicle to see if the P0121 code returns.
• Visually inspect all connections and wiring of the TPS for loose or damaged components.
• Compare the data between Circuit A and Circuit B of the TPS on the scanner and, if incorrect, perform the TPS location tests according to the manufacturer’s recommendations.

Common Errors Diagnosing Code P0121?

• Failure to follow these simple steps:
• Failure to do a visual inspection first
• No codes are scanned, documented, deleted, or retested to verify errors.
• Replacing the TPS sensor prior to testing indicates a problem.
• Not comparing TPS A and B sensor specifications before and after repair or replacement

How serious is the P0121 code?

Code P0121 will cause the engine ECM to go into degraded mode which will result in various driving ailments pending repair such as no acceleration, high idle, jerking or jolting, limited speed or stalling depending on the manufacturer degraded mode strategy .

What repairs can fix the code P0121?

1. Repair or replacement of the TPS connector
2. Repair wiring if necessary
3. Replace the TPS with a new sensor.

Additional notes regarding code P0121

• The P0121 code may be accompanied by other TPS codes, so additional repair may be required for a complete solution.
• Most of the faults for code P0121 are with a faulty TPS sensor not tracking the throttle angle properly.
• All TPS codes will cause the ECM to go into fallback mode.
• The operational symptoms for all TPS codes will be approximately the same.