Hi Everyone.

I have two different problems that I want to hear from you about.

1. Not so long ago my brake pedal started to feel a bit wrong. As if it just grabs and brakes immediately without you pressing so hard.

Later the car started to light up in abs and the red exclamation mark. But the abs light only lights up when you start the car. When you start it goes away. However, the exclamation mark continues to light up.

I put a tester on today and it says code: (C10A329) Vacuum sensor. I had the vacuum sensor replaced and reset it. But it lights up again in exactly the same way. Has anyone experienced the same thing and can give me some advice? Where else should I look?

2. Even though my car gets warm, it takes a long time to heat the cabin.

The fan doesn't run quite like it usually does either, I think.

And the passenger side blows out almost no heat compared to the driver's side.

Skoda Octavia vrs 2.0 TSi 220hp

Year 2014.

Really hope there is someone here who can guide me a little in this.

Thanks in advance and Happy New Year.

Hello there,

1) I think it looks like your master cylinder is failing. What tends to happen there is the seals fail and then fluid bypasses and gives you this binary on/off situation with the brakes.

2) With your HVAC system, it is probably either the flap motor that has failed (easy fix) or sadly (and less nice to fix) the silica bag in the header tank has burst and has made your coolant all horrible and the silica gel beads have now blocked up the heater matrix. You can check the coolant header tank. If you are lucky and have one where the silica bag can be extracted, then CAREFULLY remove it trying not to dislodge any further silica into the header tank. If it's a double skinned tank then this tank needs to be replaced with a single skin tank. THEN YOU NEED TO FLUSH the coolant system. This will take MANY hours until all the silica has been removed from the coolant system (many fills/emptying). You need some some G052188A3 coolant flush and degreaser to help here. Then fit the new header tank, fill with some G12evo coolant and then 'burp' the car by leaving the coolant header tank cap off and using VCDS/ODBEleven to purge the cooling system.

I hope that helps.

Hi friend.

First of all thank you so much for taking your time for answer my questions so good.

I checked my header tank to see if the silica is exploded. But i cannot see it. The coolant color look very suspicious. I will upload some photo of it to. How can i know if it is exploded?

If it is exploded? Should we flush thw system with this special cleaning product from VW before chaging anything? Or Should we change the header tank and more before flushing? Hope you understand what i mean. And is 5L og this cleaning product enough?

I saw several videoclips where people change the heat radiator under the console.

Is it necessary?

And for my abs problem i think you are right with the abs master cylinder. But to be sure can we check it in a special way ?

It is this one for my car right ?

Master cylinder : https://m.autodoc.dk/bosch/15086477

Photos of my header tank

I could not upload the photos in one so i upload it like this.

...

What worries me about the coolant is that here is quite a lot of crystals around the head of the tank. You sadly have a double skin tank, so you cannot get to the silica bag even if you wanted to.

So - you need a new header tank - single skin. Replace that as a first point. You also need the flush and the G12Evo coolant. Drain off the old coolant by dropping the bottom radiator hose. Flush briefly with tap water to get any initial crud. Then reconnect bottom hose, refill with the flush and then run engine back up to temperature....you will need to keep the cap off and add more flush as the level drops....also you might need to "burp" the car if the flush doesn't flow initially. Let the engine get up to temperature....switch off - let it cool - drop the flush.....then clean through again with tap water....

Its a laborous process....you need to keep on flushing until silica is removed (small little pellets of crystal)....several VAG guys I know have to do about 6 flushes before they are happy with there being no more silica in the system.

You also need to consider - you need to capture and dispose of all the liquid you use....this can be up to 40 litres of flush/dirty water/old coolant that needs to be disposed of at your local waste recycling centre.

Hi.

Tomorrow i will visit my mechanic to check.

How can i buy single skin tank? Or do you have or know the part number so i can check for exactly the single skin tank.

Where can i see it. Because it doesnt tell in the shop where i check for a new header tank.

And i will try to tell them how to clean this mess up. So we need first to get all the old coolant out first.

After this we need to use normal water to flush.

And now we use this speciel additive from VW and run the car up in temperature (with header tank cap off)

Is it right? or can you try to write it for me in sequnce. I would be so glad.

Thank you!

Edited 4 January4 Jan by vrsowner38

Your sequence is basically correct, except you need to put the header tank cap on for the special cleaner additive (because it needs to get up to temperature).

Part number is 5Q0121407AA for the header tank.

Apologies, my previous instructions were slightly incorrect:

you add it (the cleaner) to the existing coolant, run the engine to circulate the cleaner, and then drain and flush the system with clean water before refilling with new, approved coolant.

I suggest that you remove some of the coolant in the header tank first, then replace with flush. Refill the system with water, add in more flush, repeat several times.

So - here in the order:

a) First - replace the header tank, so no more silica gets into the system.

1) Remove some of the coolant in the header tank

2) Add in the VAG flush agent into the coolant header tank

3) Seal up and run up to temperature

4) Cool and then drop the coolant

5) Flush out with tap water

6) Refill with tap water and add in flush into the system.

7) Seal up and run up to temperature.

8) Repeat steps 5,6,7 a couple more times until dirt and silica beeds stop coming out of the system. VAG techs have been known to do this up to 6 times to get out all the silica.

9) Fill up with VAG G12EVO coolant (concentrate - dilute first or premade)

10) Bleed system using VCDS/ODBEleven (with header tank cap off - top off with G12evo as necessary)

That's it.

Edited 5 January5 Jan by varaderoguy

Hi again.

Thanks for your detailed explanation of the process.

I am also considering replacing my heating element as it can also get clogged. However I am having a bit of trouble finding the right part for my car. 
I called Skoda and they give me a different part number than other dealers. Is there a place to find it? 

The Skoda dealer told me it is a 

5Q0819031A

All other places I check told me it is 

5Q0819031

So where is it possible (without taking anything apart) to find the right one?

And one more question what is the disadvantage of buying a coolant reservoir without silicate? Do you have to change the coolant more often?

Thanks

On 07/01/2026 at 20:38, vrsowner38 said:

The Skoda dealer told me it is a 

5Q0819031A

Hello there; Yes - this is the correct part number. The numbering convention is model code 5Q0, then part number 819031 and then revision A.

It doesn't always work that way....its really up to the VAG part code gods how that code comes out.

On 07/01/2026 at 20:38, vrsowner38 said:

And one more question what is the disadvantage of buying a coolant reservoir without silicate? Do you have to change the coolant more often?

You should look to change your coolant every 5 years. Skoda/VAG had this "thing" that if they put in silica into header tank, it removed all the moisure out of the system as well as lubricated the cooling system. In reality, the bags split. Short answer - no difference; you just have to change the coolant every 5 years or so.

Edited 9 January9 Jan by varaderoguy

Hi.

But these two parts is different if you look up on Nissens or other companys website.

One is Denso and one in Valeo.

But both of them will fit my car it says. So i dont really know how.

And the heater is also different. I saw brunch of videos and it is different which one the each car have in it.

Ok i will order it without silikat. And then i use to G12 evo coolant only right? Because it is without silikat?

Edited 9 January9 Jan by vrsowner38

2 hours ago, vrsowner38 said:

One is Denso and one in Valeo.

Doesn't matter - it all depends on who you go with for your part. I personally would go with Denso as they are marginally better. Not much between them.

2 hours ago, vrsowner38 said:

Ok i will order it without silikat.

That's good - because nobody produces them with Silica these days anyway. The theory was a good one from VAG's perspective; the reality didn't really play out.

2 hours ago, vrsowner38 said:

And then i use to G12 evo coolant only right?

G12evo superseded G12 and G13.

Yeah but the parts is different. Look at the photos.

As long as they reference back to the original part number, you should be fine. Catalogue images can sometimes be generic.

7 hours ago, vrsowner38 said:

One is Denso and one in Valeo.

But both of them will fit my car it says. So i dont really know how.

I think only one of these will fit, it needs to match the manufacturer of the original system.

Even if you look up with VIN filtering in a genuine parts catalogue like partslink24, you get a comment "Before parts order, physical inspection of old part necessary".

If you would like me to try to look up the part number in case it is possible, tell me the VIN.

I think you can establish if the system you have is Denso or Valeo by looking at the label on the blower motor, if there are no more accessible labels than that one.

The substance in the header tank of "mit silikat'-marked tanks is silicates, not silica, and the function is to replenish levels of these chemicals that are present in the coolant when made, but get depleted during use in the engine, by chemical reactions. Not to absorb moisture, which is plentiful and desirable in coolant.

.

Edited 9 January9 Jan by Gammyleg
Misread of post above. duh!

Yeah both of them shows when i search in different places.

So it can be Valeo or Denso built. I think i will order both and send the other one back again.

Thank you guys.

Update here : Today my header tank.

And heatercore changed.

And it was a denso build. So skoda has was right about it.

But the problem is not solved. The car still not heating. So nothing changed at all.

This is the problem with partially diagnosing on a forum. Then asking someone else to do the work. Then when it doesn't solve it, who do you turn to?

I would of said, remove expansion tank, check for remnants of silicat bag with coolant emptied out. Use a light source if necessary.

If there was the bag remenant, then very likely your system is blocked you will need to fully flush not just replace the heat exchanger. Its a long, long process.. did i say long, lengthy, just checking.

If there was no remenant, then id probably go looking elsewhere. Like the flap motor. But your presumably unchanged tank, said mitt silikat.!!

Other option, if you cant do work, is ask a garage to diagnose and fix, then they are accountable!!

Edited 30 January30 Jan by TheClient
Edit added