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GM OBD Trouble Code 21

By Jim Taddei, May 1, 2009

Throttle Position Sensor Error (Signal High)

The Throttle Position sensor signals the Engine Control Module as to the current position of the engine throttle valve. The current throttle valve opening is based on the driver's input from the gas pedal. The ECM uses the information from this sensor to help calculate fuel delivery and spark timing.

Fault Code Description

Symptoms

  • Extended cranking, possible no-start condition
  • Lack of power
  • Erratic transmission operation

Common Problems

  • A failed Throttle Position Sensor (TPS) or broken wires near the sensor are the most common causes for code 21
  • Testing the TPS circuit would involve checking voltage and ground from the ECM—5v is the normal reading between the ECM reference circuit and the ECM ground circuit at the TPS connector. Checking the TPS signal circuit to the ECM would involve monitoring the TPS signal on a scan tool while jumping the 5v reference and the signal circuits. The TPS reading should go from high when the circuit is "jumpered" to low when the jumper is removed.

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