Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSSUBARU1998FORESTER L, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODESDIAGNOSTIC TESTSDTC P0122 (THV_LOW): TP SENSOR CIRCUIT LOW INPUT
1998 Subaru Forester L, Standard
DTC P0122 (THV_Low): TP Sensor Circuit Low Input
1998 Subaru Forester L, StandardSECTION DTC P0122 (THV_Low): TP Sensor Circuit Low Input
- Turn ignition off. Connect scan tool. Start engine. Observe TP sensor output signal. If displayed voltage is less than 0.1 volt, go to next step. If displayed voltage is more than 0.1 volt, system is normal. Check for an open in harness or poor contact in connectors of TP sensor and PCM connectors.
- Ensure throttle valve is fully closed. Turn ignition on. Using voltmeter connected to ground, backprobe PCM connector terminal No. 21 (Red wire). If voltage is 4.5 volts or more, go to next step. If voltage changes more than 4.5 volts while wiggling wiring and connector, check for poor contact in connectors or wiring of TP sensor or PCM connector. If voltage remains less than 4.5 volts, replace PCM.
- Using a voltmeter connected to ground, backprobe PCM connector terminal No. 6 (Light Green wire on Forester and Impreza, or White/Black wire on Legacy). If voltage is 0.1 volt or less, go to next step. If voltage is more than 0.1 volt, check voltage while wiggling wiring and connector of PCM. If voltage changes more than 0.1 volt, repair poor contact in wiring or PCM connector. If voltage remains more than 0.1 volt, go to next step.
- Turn ignition off. Disconnect TP sensor connector. Turn ignition on. Measure voltage between ground and TP sensor connector terminal No. 3 (Red wire). If voltage is 4.5 volts or more, go to next step. If voltage is less than 4.5 volts, check for poor contact in connectors or wiring of TP sensor or PCM connectors.
- Turn ignition off. Disconnect PCM connector. Check resistance of harness between PCM connector terminal No. 6 and TP sensor connector terminal No. 2. Connect one lead of ohmmeter to TP sensor connector terminal No. 2 (White wire) and the other lead to PCM connector terminal No. 6 (Light Green wire on Forester and Impreza or White/Black wire on Legacy). If resistance is one ohm or less, go to next step. If resistance is more than one ohm, check for poor contact in connectors or wiring of TP sensor or PCM connector.
- Check resistance of harness between ground and TP sensor connector terminal No. 2 (White wire). If resistance is 10 ohms or less, check for short in harness between TP sensor and PCM connector. If resistance is more than 10 ohms, check for poor terminal contact in TP sensor connector. If connection is okay, replace TP sensor.
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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.