Code 21 - ECT Sensor Malfunction

Inspect condition of connector. Clean and repair as needed. Faulty coolant temperature sensor can cause incorrect idle RPM during cold start-up condition and rich air/fuel mixture.
If no scan tester is available, go to next step. Connect and operate scan tester according to manufacturer's instructions. Read service data item No. 21. If scan tester display is approximately equal to actual engine temperature, go to next step. If scan tester display is not approximately equal to actual engine temperature, go to step 4).

Remove coolant temperature sensor. Immerse sensor in water bath. Measure sensor resistance while increasing water temperature. See COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS table. If resistance is within specification, go to next step. Replace sensor if resistance measured is not within specification.

COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS

Coolant Temperature °F (°C)	Ohms
32 (0)	5900
68 (20)	2500
104 (40)	1100
176 (80)	300

Turn ignition off. Unplug engine coolant temperature harness connector. Turn ignition on. Measure voltage at Yellow/Red wire of harness connector. If voltage is 4.5-4.9 volts, go to next step. If voltage is not 4.5-4.9 volts, repair Yellow/Red or Yellow/Green and/or Blue/Red wire between sensor and appropriate ECM harness connector terminal. See appropriate wiring diagram in WIRING DIAGRAMS section.
Test Black wire at sensor harness connector for continuity to ground. If continuity exists, go to next step. If continuity does not exist, repair Black wire to appropriate ECM harness connector terminal. See appropriate wiring diagram in WIRING DIAGRAMS section.
Temporarily substitute known good ECT sensor, then retest system. If sensor is okay, temporarily substitute known good ECM, then retest system.