dpatel20

@NikTheGeek Hard to say to be honest...my bag was not split, so not sure how the silicate was getting out (perhaps there was a small hole...I didn't see the bag after the mechanic took it out but he said it was not obviously split). That said, if it was left over sand from casting, then I would expect it to take a lot less time to become an issue. My car is at ~77K miles and I would have expected sand that was there from day one to cause an issue much sooner. The bits I took out of my expansion tank didn't feel like sand either. And, what I've read from other threads, the bags were clearly split. I have read about silicate gelation caused by having silicate in coolant systems. What I understand, silicate is used to reduce corrosion particularly in the aluminium components. But with that you risk silicate gelation. There is some info here: https://www.jstor.org/stable/44743009?seq=1 (you can read the preview to give you an idea). And this basically says that silicate gelation can block radiators, etc: https://www.frontierpower.com/service/use-low-silicate-antifreeze-heavy-duty-diesel-engines/ In my case, I think I had silicate gelation even though the bag did not split. The silicate works by coating the surfaces of the components to protect them...this can then gel up causing blockage (my interpretation/understanding of what happened in my car).

I wanted to add my experience on this topic as it might be useful for others. My issue started very much like the OP - cold air from driver's side and warm(ish) on the passenger side. Skoda Octavia MkIII, 2015. Here's my story: I first thought it would be the sensors like the OP and I tried to swap them (they're accessible on either side of the instrument panel --- if you have small hands!). Wasn't that. I read about the 'Mit Silikat' issue and saw my coolant looked very much like @centerback had in his pics: browny colour, with small bits in it. If I rubbed the bits between my fingers, they dissolved away so weren't hard lumps. I took it to my local garage and no issues seen through diagnostics (no fault codes, etc and the readings showed vent motors working fine). They flushed the system for me and that helped but not as should be - got luke warm air from both sides. I asked the garage to take the 'Mit Silikat' bag out of the expansion tank at the same time as flushing it - the bag did not seem ruptured but I presume the bits in the coolant and the browny colour was due to stuff getting out the bag. The heater matrix is located behind the centre console - I took the little trim off the side to expose the matrix pipes so I could keep an eye on them. The trim is held by one screw and just pulls down. I drove the car for a couple of weeks. Initially, the in pipe and out pipe of the heater matrix were both very hot (be careful!!) but vent air was luke warm only. So I guessed that the hot coolant was trickling through the matrix (i.e. partially blocked). After a long drive, the air went completely cool on all sides - checked the pipes: in hot, out cool. That confirmed blocked heater matrix for me. Had the heater matrix changed - around £320. About half that was the part plus some money for flush & coolant. Rest was labour. Thankfully the heater matrix can be taken out without needing to remove the whole instrument panel so only a couple of hours of labour (no charge for the diagnostics which was nice of them!). There is a video showing how to change the matrix here: https://www.youtube.com/watch?v=Ue7KP4U1xaQ. And if you search for the workshop manuals you should find they explain the same thing - see here: https://www.briskoda.net/forums/topic/461680-mk3-workshop-manual/. By the way, there are 2 different manufacturers of the heater matrix for the car (see the workshop manual). My garage were sent the wrong one first time! I did not get the expansion tank changed (should do if you want a new 'Mit Silikat' bag). I chose not to have another one of those bags - not advised but it was my choice. After that, I've had lovely hot air! Hope that helps others.