P0121 code definition
Throttle Pedal Position Sensor/Switch (TPS) A Circuit Range Performance Problem
What the P0121 code means
P0121 is a generic OBD-II code which indicates that the Engine Control Module (ECM) has seen the TPS sensor circuit A learned output voltage go above or below the sensor's expected range when compared to TPS sensor circuit B.
What causes the P0121 code?
The TPS sensor circuit A learned range output to the ECM went above the specified voltage difference between TPS sensor circuit B by more than 1.6 volts
The TPS sensor circuit A learned range output to the ECM went below the specified voltage difference between TPS sensor circuit B by going less than .8 volts.
What are the symptoms of the P0121 code?
The ECM turns the Check Engine Light on and goes into a failsafe mode by cutting current to the throttle actuator to put the throttle valve to approximately less than 6 degrees opening.
The ECM controls fuel injection and timing to control engine speeds
The vehicle would be able to be driven very slowly with limited throttle response.
The engine may idle higher than normal and not accelerate as before.
Note: The symptoms are determined by the manufacturer failsafe ECM strategy for the vehicle during a failure and may vary between manufacturers and vehicles.
How does a mechanic diagnose the P0121 code?
Scanning and documenting the codes received and viewing the freeze frame data to see when the P0121 code was set.
Clearing the OBD-II fault codes and retesting the vehicle to see if the P0121 code comes back.
Visually checks all TPS connections and wiring for any loose or damaged items.
Comparing the data between TPS circuit A and B on the scanner and if incorrect, performing the TPS pinpoint tests as per the manufacturer's recommendations.
Common mistakes when diagnosing the P0121 code?
Not following these simple steps:
Not conducting a visual inspection first
Not scanning, documenting, clearing or retesting for codes to verify failures
Replacing a TPS sensor before tests indicate a problem
Not comparing TPS sensor A and B specifications before and after repair or replacements
How serious is the P0121 code?
The P0121 code will cause the engine ECM to go into a failsafe mode, which will cause various drive complaints until repairs are done like lack of acceleration, high idle, bucking or jerking, limited speeds, or stalling, depending on manufacturer failsafe mode strategy.
What repairs can fix the P0121 code?
- Repairing or replacing the TPS connector
- Repairing the wiring as necessary
- Replace the TPS with a new sensor
Additional comments for consideration regarding the P0121 code
The P0121 code may be accompanied by other TPS codes, so other repairs may need to be followed up with in order to have a complete fix.
Most failures for code P0121 are for a defective TPS sensor not tracking the throttle angle correctly.
All TPS codes will cause the ECM to go into failsafe mode.
The drive symptoms of all TPS codes will be roughly the same.
Need help with a P0121 code?
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This article originally appeared on YourMechanic.com as P0121 OBD-II Trouble Code: TPS "A" Circuit Range Performance Problem and was authored by Jay Safford.