I've had an olde fabia (52 reg) for several months now.

Initially a cosmetic issue, I noticed the dash temperature display shows rubbish info. The shown value is ridiculously low - eg -8 on a day cool but without frost -is this normal? Does this measure come from the air intake in which case could it be sensitive to wind chill as well as actual temperature or does the sensor just need replacing? If it is a faulty sensor could it also be responsible for the following other issues

I have also noticed that the average mpg on the display has got significantly worse over the last few weeks as the weather has got colder and am wondering if the intake temperature read affects engine computer settings and a wrong value could affect fuel economy?

Another issue I've noticed is the all-or-nothing way the heater works. It is difficult to control. I've set it to a position at the moment that is reasonable but still a bit cool, but as soon as I touch it to get it warmer, it gets very hot and then turning it down seems to have little or no effect until the car has been stopped and cooled down. Is this normal and if not any ideas what is likely to be not working?

Firstly, "wind chill effect" - if this was the case - and it is not, then it would not alter the temperature sensed by a probe, it only alters the "feel" to living things as it strips out body heat quicker when the wind is cold.

 

This probe is located behind the front bumper, it should not affect any other way the engine runs as it should just be feeding the outside temperature display and probably used by the A/C if fitted to prevent it from icing up.

 

Low MPG in cold weather, yes that will happen, but it could get exaggerated by a lazy thermostat, or you demanding heating and there by slowing down the heating up of the engine.

 

Difficult cabin air temperature control, could be a faulty stepper motor, its positional feedback track could be dirty - try demanding max cold followed by max hot quite a few times, if things are now a bit better then you know what needs replacing.

Cabin temperature is semi automatic. A very simplistic semi automatic which seams to cause more confusion than its worth.

The temperature dial on your heater controls is indicating a cabin target temperature, it doesn't control a flap directly, the system does this after a calculation.

The system measures the cabin temperature via a temp sensor to the right of Fresh/recirc/AC buttons. When you select a target temperature the system decides how far to move the HVAC temperature flap/door depending on how far away from the desired target temperature the cabin actually is. Not sure if this system also measures coolant temperature as that can be part of the calculation as well.

Select a target temperature and leave it there. The system should adjust but it will take a few minutes. Don't start chasing the temperature by continually turning the dial up and down, there is a lag.

If the cabin temp sensor is blocked, covered in fluff or just plain faulty the lag can be very long indeed. Try cleaning the sensor behind the heater control grill.