VS commodore spasmodic heater and demister function

[garrex]

Hello,
I have an internal heater and demister (VS Commodore) that sometimes works and sometimes doesn't. It mostly comes on after 30 minutes of driving.
I have checked and replaced the following:
-Body Control Module
-Temperature Resistor Module (Orange cage thing) all ohm values seem fine
-All related fuses
-All earths around the car.
-Swaped relays

I didn't check the electric fan motor as it does work.
Another thing to consider is that when the heater comes on 9/10 times the demister switch will turn every thing off

Any suggestiion will be appreciated

[The1]

i had this issue with mine, and when it did work it went on and off i swapped both the relays under the dash and it still did the same thing thinking it's not the relays, i ended up going to holden and getting 2 new ones and replacing both and now it's fixed i think both mine were gone.

[gregu78]

I recently had the same problem, replaced blower fan, climate control unit, checked everything. Ended up being a relay under the bonnet labelled "L/P Check" - Next to the fuel pump relay.

[hako]

You say the fan works OK so it should not be an electrical problem, rather I'd figure on it being a vacuum problem as it's vacuum that operates the flaps which direct/control hot or cold air depending on what temp you select. Vacuum also controls the heater tap which turns the flow of hot water to the heater on and off. The only thing that's electric is the fan....and that's OK.
First I'd do some detective work....has a new head unit been installed recently or any work under the dash been carried out?...if so it is very easy to dislodge a vacuum hose which then reduces drastically the vacuum available to operate the vacuum motors that control the flaps etc. When you have turned the heater on but are getting cold air, feel the brass pipes visible just above your left ankle when driving - they should be hot and if not this means the heater tap is not operating. See attached pix of pipe location.