The heater on my 360 started blowing constantly hot over the weekend, regardless of what setting I had put the controls to. So it's blowing hot even when on the cold setting, and also with the aircon on.

Obviously there's a fault somewhere, what would the triage approach be on diagnosing a problem like this? Where would I look first, starting with the simplest possible explanation?

You car is a GLEi from memory... has it got air con?

If not, underneath the heater box in the bonnet (big black plastic thing under windscreen) there is a valve on a water pipe. You can turn it by hand fairly easily, it's most likely the cable that operates this valve thats jumped off. If you turn it by hand you can shut the valve, then if it still blows hot the valve is stuffed. If it blows cold, the cable operating it has gone walkies.

If it has aircon, I've no idea how much of the system is the same!

Yes, GLEi and it has aircon!

I'll have a squint later, see if it resembles your description.

Cheers.

That part of the system is entirely the same, A/C or no A/C. The other possibility is that the cable has jumped off or bent up at the other end, where the lever for adjusting the temperature is. I've seen that a few times too, particularly where the cable has rusted up and some numpty has just forced the lever over with no thought and bent it all up. I hope that's not the case because its a PITA to get to and almost impossible to straighten the cable properly again.

I've bent both the cables on my cars, if you don't look after them or the valve then the force will become too much for the cable.

Silver steel wire plus good forming pliers (not tool box pliers ) and you are going again.

This is getting weirder.

Today it's blowing cool on the cold setting, and the aircon being on isn't making it any cooler.

I had a look at that valve on the water pipe, and frankly it all looks and feels pristine.

I think I'll need to get a garage to give it all the once over, including the aircon.