Coolant Temperature Sensor/Switch (For Computer): Testing and Inspection

Coolant Temperature Sensor Schematic:

A faulty coolant temperature sensor will set code 21 in the on-board diagnostic system. Refer to the schematic diagram and test the coolant sensor with the diagnostic chart.

Wire color code identification:

L: Blue
B: Black
Y: Yellow
G: Green
R: Red
W: White
Br: Brown
Lg: Light green
Gr: Gray

Coolant Temperature Sensor Diagnostic Chart:

COOLANT TEMPERATURE SENSOR DIAGNOSTIC CHART

ECU/Coolant Temperature Sensor Harness Testing:

CHECK ECU VOLTAGE

1. Turn ignition "ON" (engine "OFF").
2. Check ECU voltage between the following connector terminals:

Connector: Terminal: Voltage:
F121 5-1 0.15-4.85V
F121 5-2 0.15-4.85V

CHECK HARNESS BETWEEN ECU AND TEMPERATURE SENSOR

1. Disconnect ECU connector.
2. Measure resistance between ECU connector and sensor connector:

Connector & terminal: Resistance:
(F121)5-(F22)2 Below 1 Ohm
(F121)1-(F22)1 Below 1 Ohm
(F121)2-(F22)1 Below 1 Ohm

3. Measure resistance between ECU connector and terminal (F121)5 and ground. Reading should be infinite.

Coolant Temperature Sensor Testing:

CHECK COOLANT TEMPERATURE SENSOR

1. Disconnect coolant temperature sensor connector.
2. Measure resistance between sensor terminals 1 and 2 (sensor side):

Resistance: Temperature:
2 - 3K Ohms 20° C (68° F)
270 - 370 Ohms 80° C (176° F)