P0121 - Throttle Position Sensor/Switch A Circuit Range/Performance Problem

The TPS is a potentiometer used to indicate how much acceleration the driver needs. This has a 5-volt reference power and emits a ground signal, which comes from the PCM (Powertrain Control Module).

The sensor is used by the PCM to control both the delivery of fuel, and to operate the automatic transmission. A general measure is: At rest: 0.5 volts; and at full speed: 4.5 volts. If the PCM verify the angle of this accelerator is less or greater than that needed for a specific RPM (Revolutions Per Minute), the DTC code P0121 is established.

The symptoms of a code P0121 could include:

• Check Engine service light is on.
• There may be irregularities in the acceleration / deceleration.
• When accelerating, black smoke comes out of the exhaust pipe.
• The vehicle may not start.

A code P0121 can be caused by the following problems:

• There are open lines in the sensor connection to the PCM.
• There are readings that are out of range.
• The TPS sensor may be damaged.
• The PCM could have failures.
• The acceleration system may be failing.

• With a scan tool, check the readings that give the idle and Wide-Open Throttle, to the TPS (Throttle Position Sensor) and check if they are within the specifications mentioned. If that is not the case, replace the TPS and verify again.
• With an oscilloscope There must be no short circuit in the TPS signal, there should be no abandonment.
• Move the TPS connector and the harness, at the same time, see the pattern and if it is abandoned you could substitute the TPS and recheck.
• Check that there is 5 volts of reference in the TPS connector, if so, verify the circuit to ground.
• There may be a short circuit, to check it make sure that the signal circuit is not 12 volts.
• Verify that the connector has no water, if so, you could replace it.