Testing the coolant temperature sensor of Fiat Punto

1 The sensor is located at the left-hand end of the inlet manifold, and can be identified by its green wiring connector (see illustration).

Engine coolant temperature sensor location (arrowed) on the left-hand end of the inlet manifold

2 The unit contains a thermistor - an electronic component whose electrical resistance decreases at a predetermined rate as its temperature rises.
3 The engine management ECU in Fiat Punto supplies the sensor with a set voltage and then, by measuring the current flowing in the sensor circuit, it determines the engine temperature. This information is then used, in conjunction with other inputs, to control the engine management system and associated components. The sensor signal is also used to operate the temperature gauge and/or warning light on the instrument panel.
4 If the sensor circuit should fail to provide plausible Information, the ECU back-up facility will override the sensor signal. In this event, the ECU assumes a predetermined setting which wilt allow the engine management system to operate, albeit at reduced efficiency. When this occurs, the engine warning light on the Instrument panel will illuminate, and the advice of a Fiat dealer should be sought. The sensor itself can be tested by removing it, and checking the resistances at various temperatures using an ohmmeter (heat the sensor in a container of water, and monitor the temperature with a thermometer). The resistance values of Fiat Punto sensor are given in the Specifications.


6 Remove the air cleaner assembly.
7 Remove the battery and battery tray.
8 Partially drain the cooling system to just below the level of the sensor. Alternatively, have ready a suitable bung to plug the aperture in the housing when the sensor is removed.
9 Disconnect the wiring connector from the sensor.
10 Carefully unscrew the sensor and recover the sealing ring. If the system has not been drained, plug the Fiat Punto sensor aperture to prevent further coolant loss.


11 Check the condition of the sealing ring and renew it if necessary.
12 Refitting is a reversal of removal, tightening the sensor to the specified torque.
13 On completion, start the engine and run it, until it reaches normal operating temperature.
Continue to run the engine until the cooling system in and out correctly.