Technical Description of OBD-II Trouble Code
Malfunction in Circuit A of Throttle Position Sensor/Switch (TPS)
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to vehicles equipped with OBD-II. Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
The TPS (Throttle Position Sensor) is a potentiometer that is mounted to the throttle body. Its function is to detect the angle of the throttle blade. As the throttle blade moves, the TPS sends a signal to the PCM (Powertrain Control Module). The TPS is usually a three-wire sensor, with a 5-volt reference from the PCM to the TPS, a ground from the PCM to the TPS, and a signal return from the TPS to the PCM.
The TPS sends the throttle position information back to the PCM through the signal wire. When the throttle is closed, the signal voltage is approximately 0.45 volts. At WOT (Wide Open Throttle), the TPS signal voltage will reach close to 5 volts. If the PCM detects a voltage that is outside the normal operating range, it will set the P0120 code.
NOTE: The PCM recognizes that any significant change in throttle position corresponds to a change in manifold pressure (MAP). In some models, the PCM will monitor the operation of both the MAP and TPS for comparison. This means that if the PCM detects a large percentage change in throttle position, it expects to see a corresponding change in manifold pressure, and vice versa. If it does not observe this comparative change, it may set the P0120 code. However, this does not apply to all models.
Symptoms
Possible symptoms include:
Causes
Possible causes of the P0120 code include:
Possible Solutions
If you have access to a scan tool, observe the TPS voltage with the engine off and the key on (KOEO). With the throttle closed, the voltage should be around 0.45 volts. As you depress the throttle, the voltage should gradually increase to approximately 4.5 to 5 volts. Sometimes, only a scope can capture an intermittent glitch in the TPS signal voltage. If you notice a glitch in the TPS sweep voltage, replace the TPS.
NOTE: Some TPS sensors require fine adjustment. If you are not comfortable using a Digital Volt Ohm meter (DVOM) to adjust the new TPS, it is best to take the vehicle to a professional shop. If the voltage is not approximately 0.45 volts (with a tolerance of around 0.3 volts) when the throttle is closed, or if the reading is “stuck,” then unplug the TPS connector. With the engine off and the key on (KOEO), check for the presence of a 5-volt reference voltage and a good ground at the TPS connector. You can check the signal circuit for continuity by connecting a fused wire between the ground circuit of the TPS connector and the signal circuit. If the TPS reading on the scan tool now reads zero, replace the TPS. However, if that does not change the reading to zero, check for an open or a short in the signal wire. If no issues are found, suspect a faulty PCM. If manipulating the TPS wiring harness causes any change in idle, suspect a faulty TPS.
Other DTCs related to TPS sensors and circuits: P0121, P0122, P0123, P0124
