There are normally two temperature sensors on a fuel injected engine, one for the coolant temperature and one for ambient air temperature. These are needed by the ECU to decide how much fuel to inject, the mixture needs to be rich to ignite at lower temperatures ( more fuel for a given amount of air ). Sometimes a failing engine coolant sensor may not trigger a fault light or code. The coolant temp sensor is fairly cheap - £15 to £20, it screws into engine block and the sensor part is in the coolant. On MK1 Jazz the sensor is in rear of cylinder block at gearbox end, may need to remove air cleaner to access it, it is right next to crankshaft position sensor, the temperature sensor actually screws in the engine and the crankshaft sensor has single bolt fixing. Don't know if it is possible to see if the sensor is working via OBD reader or need to check voltage output when cold and hot with voltmeter. I think Jocko could see the engine temp on his plug in permanently plugged in OBD that he kept on dashboard for MPG etc. An OBD scanner gives a lot more info, but you pays yer money and gets what you need.
