Code 3, Coolant Temperature Sensor

1. Turn ignition to "OFF" position. Unplug CIS-E control unit connector. Check resistance between sockets No. 7 and No. 21. Resistance reading should be as indicated in table. See Fig 1 . If not okay, check wiring.
2. Unplug coolant temperature sensor connector. Measure resistance between any 2 of the diagonally opposed terminals and compare resistance values from each internal sensor. Resistance reading should be as indicated in table. See Fig 1 . If not okay, replace sensor.
3. Check wiring harness continuity to CIS-E control unit. Resistance should be less than one ohm. If not okay, check for open circuit.

Fig 1: Coolant Temperature Sensor Resistance

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.