1994 Subaru Legacy AlpineFACTORY OEM SERVICE PROCEDURE

Code 31 - Throttle Position Sensor

NOTE: Refer to illustration for ECU connector and terminal identification. See Figure .
  1. Turn ignition on. Measure voltage between ECU connector B58, terminal No. 2 (backprobe) and ground with throttle fully open and fully closed. With throttle open, .7-1.6 volts should be present. With throttle closed, 4.4-4.8 volts should be present. Voltage should smoothly increase/decrease as throttle is opened/closed. Backprobe ECU connector B58, terminals No. 1 and 3 with voltmeter to ground. Terminal No. 1 should be zero volts. Terminal No. 3 should be 5 volts.
  2. If voltages are to specification, check ECU terminals for poor contact. If terminals are okay, replace ECU. If voltages are not to specification, go to next step.
  3. Turn ignition off. Disconnect throttle position sensor connector. Measure resistance between throttle position sensor terminals No. 2 (Black wire) and No. 3 (Red wire). Ohmmeter should indicate 12,000 ohms.
  4. Measure resistance between throttle position sensor terminals No. 2 (Black wire) and No. 4 (White wire). With throttle fully closed, 10,000-12,000 ohms should be present. With throttle fully open, 3000-5000 ohms should be present. If resistances are not to specification, replace throttle position sensor. If resistances are to specification, go to next step.
  5. Disconnect ECU and throttle position sensor connectors. Measure resistance between ECU connector B58, terminal No. 1 and throttle position sensor connector terminal No. 2 (Black wire). Measure resistance between ECU connector B58, terminal No. 2 and throttle position sensor connector terminal No. 4 (White wire). Measure resistance between ECU connector B58, terminal No. 3 and throttle position sensor connector terminal No. 3 (Red wire). Zero ohms should be present in all cases. If resistance readings are not zero ohms, repair harness as necessary.
  6. Measure resistance between throttle position sensor connector terminals No. 2 (Black wire), No. 4 (White wire), No. 3 (Red wire), and ground. Minimum of one megohm resistance should be present in all cases. If minimum of one megohm resistance is not present, replace throttle position sensor.
  7. If resistances are to specification, repair poor terminal contact at ECU. If terminal contact is okay, replace ECU.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.