Vauxhall Meriva my car is coming up with code 4.

First thing to check would be the coolant level . If o.k it may be a faulty thermostat that is sticking open .On a lot of cars the air con comes on automatically to clear screen quicker when the sensor triggers it

Thanks for the reply. I will check the coolant level, etc. The air con hasnt automatically come on in the past, the heater normally clears the windscreen. So will check all above, Thank you.

This is the type of sensor that triggers the air con depending on conditions whether it's needed or not . If you dont have it ,( climate control sensor ) doesn't apply to your car .

https://www.youtube.com/watch?v=QxvxcPiDK0s