Code 21: Coolant Temperature Sensor

NOTE: Refer to illustration for ECU connector and terminal identification. See Figure .

Turn ignition on. Backprobe ECU connector F121 and measure voltage between terminals No. 1 and 5 and between terminals No. 2 and 5. If voltage measurement at each terminal is 0.15-4.85 volts, go to next step. If either voltage measurement is not 0.15-4.85 volts, check for faulty wiring harness or poor ECU connector contact. If wiring harness and connectors are okay, replace ECU.

Turn ignition off. Disconnect coolant temperature sensor connector. Measure resistance between coolant temperature sensor terminals. See COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS table. If resistances are not as specified, replace coolant temperature sensor. If resistances are as specified, go to next step.

COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS

Temperature °F (°C)	Ohms
68°F (20°C)	2000-3000
176°F (80°C)	270-370

Disconnect ECU connectors. Measure resistance between ECU connector F121, terminal No. 5 (backprobe) and terminal No. 2 (White/Black wire) of coolant temperature sensor connector. Measure resistance between ECU connector F121, terminal No. 1 (backprobe) and terminal No. 1 (Black/Green wire) of coolant temperature sensor connector. Also, measure resistance between ECU connector F121, terminal No. 2 (backprobe) and terminal No. 1 (White/Black wire) of coolant temperature sensor connector. If all resistances are less than one ohm, go to next step. If any resistance is more than one ohm, repair connector and/or wiring harness as necessary.
Measure resistance between ground and ECU connector F121, terminal No. 5 (backprobe). If resistance is infinite, no problem has been found. Check for poor ECU connector contact. If connectors are okay, replace ECU. If resistance is not infinite, repair connector and/or wiring harness as necessary.