Last week when I pressed the windscreen demist switch, no air was directed to the screen. I tried the 'up arrow' switch but still no joy. I dropped the car off at my local garage for them to look at the next day. However, when they checked the car, it worked perfectly.

Yesterday, in the pouring rain driving to and from Manchester, the demister failed to direct air to the windscreen again. It was like driving in fog, but I was the only one effected. Again, I called my local garage, but this time they can't see the car until the New Year. However, when I tried the demister this morning, everything work perfectly.

Has anyone any idea about what the problem might be. The garage can only fix a fault that exists at the time.

Electric flap motors are known to be prone to failure, but I am unsure if your car is of an age to have electric flaps or mechanical controls.

No electric flaps on this age of car Cliff.

Yet you mentioned a switch not a lever.

https://ebay.us/m/C82cMh

If you open the link, you will see the panel with the demister switch bottom left and the 'up-arrow' switch upper right side.

Maybe button would have been a better description than switch.

There are no mechanical connections on that unit so demisting must be electric. Are you sure you don't have embedded electric filaments in your windscreen, which of course do not control air flow?

This is all I can find on the system on YouTube. I don't know what could be intermittent here. I wonder if the panel could be the culprit?

That video confirms electric flaps, and tells you all you need to know.

I guess the question is what could cause an intermittent fault. The panel or the flap motor? Will this system give fault codes on a 2003 car?

4 hours ago, cliffcoggin said:

That video confirms electric flaps, and tells you all you need to know.

Thinking about the 2013 Golf I had previously, the symptoms are very different. That would start the journey with everything working fine, but gradually degraded after driving for a couple of hours. The A4 is all or nothing right from turning the start key.

2 hours ago, greggles said:

Thinking about the 2013 Golf I had previously, the symptoms are very different. That would start the journey with everything working fine, but gradually degraded after driving for a couple of hours. The A4 is all or nothing right from turning the start key.

Hi have you changed the pollen filter recently?

Steve.

Hi Steve, checked with my local garage that completed the 150,000 mls service last March. They stated that the pollen filter would have been removed, cleaned and refitted.

What I am really interested in is whether this system is monitored so it might give fault codes.

Previously, the windscreen demister had failed, on both occasions it was on a Thursday. However, two Thursday's have past now without incident, therefore, I can deduce that Thursday is not the issue here! However, I have made a decision regarding this problem. Previously, in a seperate post, I asked a question about changing the glove box brake/switch unit. Turns out the glove box does need to be removed. Therefore, there will be access to the demister flap motor, which will be changed speculatively. My thoughts on the panel not being an issue are, there are two different buttons to select air to demist the windscreen. If one or the other didn't send a signal to the demister flap motor, then the panel would definitely be the faulty item. Anyway, time will tell I suppose.

Hi Steve, it turns out my car can have it's heating and ventilation system checked for a fault code.

The test showed a windscreen motor failure. Because the glovebox had to be removed and it had a faulty light switch/brake, I had this changed at the same time.

How the windscreen demister works everytime and the glovebox opens with a dampened movement, plus the light comes on too.

Happy days!!!!