Fault code P0121 – a step-by-step guide to diagnosing and fixing it

DTC P0121 indicates that the output voltage of the car's throttle position sensor (TPS) circuit “A” is above or below the normal range. The engine control module (ECM) uses the sensor’s readings to determine how open the throttle is and compares the circuit A voltage to that of circuit B. As the throttle opens, the voltage should increase. At full throttle the output is normally around 4.5 V.

• It could be due to a fault with the TPS itself. If the device is installed incorrectly, it could fail and affect the car's engine performance.
• An open or short sensor circuit. This could be an intermittent or internal fault. The sensor could be affected by wire damage, contamination or loose connections.
• Damage to the TPS housing. Water or contaminants could get into the device and cause the components to corrode.
• A bad connector. If the connector is corroded or broken, it may result in a bad connection to the control module.

• Activation of the check engine light on the dashboard
• Rough idling
• Difficulty starting the engine
• Sudden stalling
• Sluggish acceleration or deceleration
• Black exhaust fumes
• Misfiring when the engine is idling or operating at high RPMs

• First of all, you need to diagnose the problem. Connect an OBD2 scanner or similar scan tool to the vehicle via the data link connector port. You can use this tool to scan for DTCs and check the freeze frame data to see when the trouble code was set. This data will help you to pinpoint the cause and identify underlying problems. Reset the DTCs and scan the computer again later.
• If the DTC reappears, inspect the body of the TPS device and wires for signs of damage, such as cracks, traces of melting, frayed insulation, or severe corrosion. Disconnect the connectors and examine the contacts for contamination or rust. Replace any components that are visibly defective. It may be possible to restore the connectors by removing traces of corrosion using a special cleaning agent.
• While the ignition is off, disconnect the sensor wiring harness. Then turn the ignition key to the on position without cranking the vehicle. Check the reference voltage to the sensor using either a voltmeter or digital multimeter. Follow the vehicle manufacturer’s instructions when carrying out this test. The meter reading should be about 5 V. You should also be able to check the signal voltage with this tool. While the car's throttle body is closed, this should be between 0.2 V and 1.5 V depending on the model. If no voltage is detected, it is likely that there is a short or a problem with the harness. If the readings are not within the manufacturer’s specifications when the throttle is open or closed, this is a sign that the TPS needs to be replaced.