Help!
My car has had a coolant leak so got a mechanic out to have a look. He couldn’t find any leak, pressure test showed losing pressure, no fluid anywhere, inside footwells dry and a fault code showing ‘p199e cylinder head coolant valve open circuit’

Other Info - car heats up fast, no hot air coming from vents, car doesn’t overheat but sits at 90.

2013 2.0 Tdi mk7 golf bluemotion.

I’ve read somewhere it could be some pressure sensor / converter exhaust control… really don’t know tbh.

Any advice? Is this a common fault!?

Thanks in advance