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.
1994 Subaru Legacy AlpineFACTORY OEM SERVICE PROCEDURE
Code 21 - Coolant Temperature Sensor
NOTE:
Refer to illustration for ECU connector and terminal identification. See Figure
.
- Turn ignition off. Disconnect coolant temperature sensor connector. Measure resistance between coolant temperature sensor terminals. See the COOLANT TEMPERATURE SENSOR RESISTANCE
table. If resistances are not to specification, replace coolant temperature sensor. If resistances are to specification, go to next step.COOLANT TEMPERATURE SENSOR RESISTANCE
Temperature °F (°C) Ohms 68°F (20°C) 2000-3000 176°F (80°C) 300-400 - Turn ignition on. Measure voltage between ECU connector B48, terminal No. 7 (backprobe) and ground. If .6-4.5 volts are present, check ECU terminals for poor contact. If .6-4.5 volts are not present, go to next step.
- Ensure ignition is off. Disconnect ECU and coolant temperature sensor connectors. Measure resistance between ECU connector B48, terminal No. 7 and coolant temperature sensor terminal No. 1, then measure resistance between ECU connector B48, terminal No. 21 and coolant temperature sensor terminal No. 2. Zero ohms should be present in both cases. Repair harness if not zero ohms.
- Measure resistance between coolant temperature sensor harness terminals and ground. At least one megohm should be present. If resistance is to specification, repair ECU terminals for poor contact or replace ECU. If resistance is not to specification, repair harness and/or connector.
Factory Service Tools
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
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.