The throttle position sensor (TPS) is used by the electronic control module (ECU) to govern engine speed. The ECU monitors the voltage from the potentiometer and the output of other sensors to determine exact engine. If there is a problem with the TPS, the ECM will log a TPS Trouble Code. Trouble code P021 is an indication that the ECM cannot determine the signal from the throttle position sensor.
Open the hood and set the prop rod to hold the hood in the open position. Turn the key switch "Off." Disconnect the negative battery cable from the negative battery post. Remove the positive battery cable from the positive battery post. Visually inspect the engine harness and look for any damage, corrosion or abrasion to the wires in the engine harness. Temporarily repair any broken wires with black electrical tape. Use baking soda, water and a stiff brush to remove from a terminal connection. After repairs are complete, connect the positive battery cable to the battery. Reconnect the negative cable to the battery.
Use a digital voltmeter to measure the voltage at the throttle position sensor connector. Most throttle position sensors are powered by 5 volts. Refer to the engine schematic for details and locate the V+ wire to the TPS. Turn the key switch "On." Measure the voltage from V+ to sensor common or chassis ground. The voltage should be approximately 5 volts. If the voltage is less than 5 volts, there is a problem in the wires from the ECM to the TPS.
Turn the key switch "Off." Measure the resistance of the V+ wire from the TPS connector to the ECM connector. The resistance should be less than 5 ohms. If the resistance is greater than 5 ohms, there is a problem in the V+ wire. Replace the wire. Measure the resistance of the sensor common wire from the TPS connector to the ECM connector. The resistance should be less than 5 ohms. If the resistance is greater than 5 ohms, there is a problem in the sensor common wire. Replace the wire. Measure the resistance of the sensor signal wire from the TPS connector to the ECM connector. The resistance should be less than 5 ohms. If the resistance is greater than 5 ohms, there is a problem in the sensor signal wire. Replace the wire.
Connect the TPS to the TPS connector. Use a wire removal tool and remove the TPS signal wire from the TPS connector. Turn the key switch "On." Use a digital voltmeter to measure the voltage output of the TPS. Place the positive probe on the signal wire and place the negative probe on chassis ground. Move the TPS while measuring the voltage. The voltage should be approximately 0.5 volts when the throttle is in the low position or low idle position. Move the throttle to the Wide Open Throttle position, the voltage should gradually change from 0.5 to 4.5 volts. If the voltage is not correct, replace the throttle position sensor.
