Octavia 2021 blowing hot air on the driver side (all vents), driver side rear feet and rear middle. On passenger side it doesn't blow really hot on the front middle vent and blows only cold on the right side vent and rear foot vent.

First the passenger side middle vent was blowing freezing like the others on passenger side but after couple of hours of driving it has started to blow littlebit warmer, but the others are still blowing really vold air.

The problem started in the morning when the heater was turned max heating, car has climatronic and outside temperature was -32 celsius.

Is there any common problems that this could be caused by?

At -32C you may be experiencing coolant freezing up. Or at least getting solid enough not to move around the pipes. Get a heated garage!

On 04/01/2026 at 01:06, inspectorman said:

At -32C you may be experiencing coolant freezing up. Or at least getting solid enough not to move around the pipes. Get a heated garage!

Yeah. I have had the car in the garage now for a week and still not blowing hot. I removed the glove box and tried to check the motors that adjusts the flaps but they seem to work perfectly.

If the coolant reaches the correct temperature properly, then touch the coolant hoses in the engine room that go to the heater core. If the inlet is hot and the outlet is only warm, then the heater core is likely clogged.

1 hour ago, ten4doors said:

If the coolant reaches the correct temperature properly, then touch the coolant hoses in the engine room that go to the heater core. If the inlet is hot and the outlet is only warm, then the heater core is likely clogged.

Thank you. Just did this and other was hot and other warm. Removed the heater core and it was half full of simething brown. Tried to wash it but most of it is stuck there. I have to order a new core and hopefully it helps.

AFAIK, it should not be the case for the Mk4 - but there was an issue with the 'Mit Silicat' insert in the coolant expansion tank of earlier Mk3 models splitting, causing blockages, especially in the heater matrix.

13 minutes ago, Joni5629 said:

Thank you. Just did this and other was hot and other warm. Removed the heater core and it was half full of simething brown. Tried to wash it but most of it is stuck there. I have to order a new core and hopefully it helps.

How difficult is this replacement on an Octavia 4? I have the 1.0 TSI 81 kW. I’m thinking about doing this job myself too, but I haven’t yet found any good Octavia 4 video guides.

1 hour ago, ten4doors said:

How difficult is this replacement on an Octavia 4? I have the 1.0 TSI 81 kW. I’m thinking about doing this job myself too, but I haven’t yet found any good Octavia 4 video guides.

Easy. Just be careful eith the pipes so they dont start leaking or break. When you take the coolant pipes you should try to prevent the coolant from leaking to the floor. I put a box under the haeter core and catch most of the coolant.

First you should open the hood and somehow squeeze the coolant hoses that goes in so all the coolant doesn't come to the floor of the car. I used two locking pliers and paper between the pliers and hoses.

Remove the lower plastic cover on the left side of the center console. It has one torx bolt and then just pull it down.

After that you have to remove the air vent that blows to the legs of the driver. It has one torx that you can barely see when looking in the little gap under the steering wheel and then just pull the air vents of.

Now you can see the heater core. Remove the two clamps with a very little torx and very little space to move. Then there is little clip with torx that holds the core, remove it and now you can pull the core off. Just move the pipes a little and you can pull the core. The heater core clamps should be renewed in the process.

2 hours ago, Warrior193 said:

AFAIK, it should not be the case for the Mk4 - but there was an issue with the 'Mit Silicat' insert in the coolant expansion tank of earlier Mk3 models splitting, causing blockages, especially in the heater matrix.

Yeah it looked like rust but is there anything other than the engine block that could rust?

I believe engine block and head are aluminium.

Has the coolant concentration been maintained in service records?

1 hour ago, Warrior193 said:

I believe engine block and head are aluminium.

Has the coolant concentration been maintained in service records?

I don't remember but I can do it. It should ve over -35celsius, otherwise they should have mentioned it.

I replaced the heater core. Since I couldn’t find a video about this specific combination (Octavia 4 + 5WA819031), here are a few notes to share with others:

a) Most shops sell a heater core for my car with the code 5Q0819031, and I even managed to order one before realizing that the correct part is 5WA819031. Physically check which heater core is actually installed in your car before ordering.

b) Look for the screw for footwell air duct on the right of the steering column (on my left-hand-drive car).

c) The plastic part of the center console gets in the way. I bent it aside.

d) The clamp screws are very difficult to access. When installing the new heater core, it would be wise to position them in a more accessible way to make the job easier next time.

Compared to the Octavia III, there is significantly less space to work with.