VEHICLE

A vehicle (10) is disclosed which includes a thermostat (22) in a bottom hose (24) of a radiator (16). An onboard diagnostic arrangement is provided to monitor the efficiency of the thermostat (22). Temperature sensors (S1, S2) are provided to monitor the temperature of the coolant across the radiator (16) so that a controller (40) can derive from those measurements the temperature differential across the thermostat (22). If the temperature Teng of the coolant at the thermostat inlet is below the thermostat opening temperature Topen and the differential temperature between the thermostat inlet temperature Teng and the thermostat outlet temperature Trad is below a fault threshold Dtstat, the thermostat (22) is evaluated as being stuck open or leaking, which results in a fault being recorded and a malfunction indicator lamp (MIL) being illuminated.