The cabin heater box contains a hidden temperature sensor that the car uses to control the air conditioning.
It cross references the heat reading from this sensor and the IR sensor in the heater controller so it can decide
how much the air conditioning needs to run and at what temperature to maintain the set cabin heat that is selected.

From late 2013, these AC temperature sensors started to fail and look like they'll become a new common fault.

The Evaporator Temperature Sensor

This is the sensor causing problems. It's enormous! About 2 feet long. OK, that's a lie, it's tiny.



The Symptoms

Very similar to having low AC gas. The AC light will come on as usual. The compressor won't kick in and no cold
air comes from the vents inside the car. So, if you have checked the gas pressure and it's OK, check this sensor.

Removing The Sensor

Look in the right hand footwell towards the centre of the car.



You'll see a connector with 2 wires coming from a raised part of the cabin heater duct.



To remove it, you just need to pull.



Disconnect the electrical connector by pulling on the plug (not the wires).



When you refit the sensor, it goes in facing down. It just clips in as before.

They are available on eBay or from smart using smart part number A 220 830 07 72 (I think).

Bench Testing

If you have a multimeter you can test your existing sensor to see if it works. See here.

Fooling The System

You can guarantee that this sensor will fail on the hottest day of the year when you absolutely must have AC on.
Instead of waiting for one to be delivered or having to go to smart and hope they have one, you can trick it.

The car is just looking for a resistance to tell it how warm the air is. At a guess I'd say that pushing a 2KΩ resistor into the wiring connector would be enough to make the car think it's about 30 degrees celsius. This should be about right to get it working. You'll have to experiment as the sensor has a wide resistance output.