Hi, has anyone had this fault,my skoda octavia 1.9tdi engine code bkc has developed a heating issue! If you drive at speed the heaters go from hot too cold once you slow down  to around 30mph ish! If drive for awhile the heaters go hot again! also if you just drive at low speed the heaters are hot! I've replaced the thermostat,water pump,done a block test and flushed the system out with a hose pipe! Anyone had this fault? 

Going by the age of the vehicle you are probably looking at oxidised connections to the various vent temperature sensors including bad solder joints to the interior temp sensor and sunlight sensor.

 

A diagnostic tool showing live data from each sensor will clarify this.

Hi, this fault only comes on if you put your foot down hard and then slow down! Can't see how these sensor would cause that? 

Watch the level in the coolant tank as you rev the engine. Does the level go down? Is the level correct when idling?