As the weather has cooled somewhat I’ve noticed the heaters in my 1.5 TSI seem a bit lukewarm. Even set to “Hi” after warming up at 4 degrees external temperature the air is cool.

Now the strange thing is that over the course of a working day the car does get warmer inside and doing checks with a temperature probe revealed that at “25” the air is 30/25/27/28 degrees from the four vents left to right. Stuck in motorway traffic the heater temperature does get pretty warm but then cools again under normal city driving. Surely if the car is at operating temperature the heaters should be piping hot at “Hi” under any driving conditions?

Is this the way these heaters work normally or is something wrong here. They didn’t get particularly warm this morning at 4 degrees.

My mechanic suspects the waterpump and as the car Is on 155k it could be due a change. However there’s no failing waterpump evidence like overheating or noises.

The other possibility is the matrix so it’s booked in for a flush and possible replacement as an initial diagnosis.

2 hours ago, gm73 said:

As the weather has cooled somewhat I’ve noticed the heaters in my 1.5 TSI seem a bit lukewarm. Even set to “Hi” after warming up at 4 degrees external temperature the air is cool.

Now the strange thing is that over the course of a working day the car does get warmer inside and doing checks with a temperature probe revealed that at “25” the air is 30/25/27/28 degrees from the four vents left to right. Stuck in motorway traffic the heater temperature does get pretty warm but then cools again under normal city driving. Surely if the car is at operating temperature the heaters should be piping hot at “Hi” under any driving conditions?

Is this the way these heaters work normally or is something wrong here. They didn’t get particularly warm this morning at 4 degrees.

My mechanic suspects the waterpump and as the car Is on 155k it could be due a change. However there’s no failing waterpump evidence like overheating or noises.

The other possibility is the matrix so it’s booked in for a flush and possible replacement as an initial diagnosis.

I can’t seem to edit this long winded post so I’ll do it in this quote.

Quite simply I’m getting decent heat when set to about 25 and blower on the lowest setting. When I turn it up it goes cold.

Matrix or waterpump?

Would anyone be able to hazard a guess?

Many thanks.

Before you head down that rabbit hole, can I suggest you check the external temperature sensor has not failed (a common failure) and possibly the rain/humidity sensor under the mirror has also not borked. MK3 (not the facelift 3.5's) - had a dodgy humidity sensor that failed (the one below the rear view mirror) and causes the fan speeds and temperature settings not to work properly.

A quick scan of codes would see confirm if that was the issue.

re: waterpump. If the car is coming up to temperature and sits nicely at 90C, then it probably isn't the waterpump. MK3 (not 3.5's) had the dodgy plastic impellers that fail. Usual signs are a overheat, then recovery, then complete failure. If you've had a new waterpump since 2018, it will probably be the new type that don't fail. Don't forget there is also a secondary coolant pump in the cooling system that is used especially if you have stop/start.

Other things to check: have you removed the silica bag of doom in the coolant header tank? If this splits, it will get into the heater matrix and stop it working (it will also get into the engine block too, so that will need urgently addressing).

18 minutes ago, varaderoguy said:

Before you head down that rabbit hole, can I suggest you check the external temperature sensor has not failed (a common failure) and possibly the rain/humidity sensor under the mirror has also not borked. MK3 (not the facelift 3.5's) - had a dodgy humidity sensor that failed (the one below the rear view mirror) and causes the fan speeds and temperature settings not to work properly.

A quick scan of codes would see confirm if that was the issue.

re: waterpump. If the car is coming up to temperature and sits nicely at 90C, then it probably isn't the waterpump. MK3 (not 3.5's) had the dodgy plastic impellers that fail. Usual signs are a overheat, then recovery, then complete failure. If you've had a new waterpump since 2018, it will probably be the new type that don't fail. Don't forget there is also a secondary coolant pump in the cooling system that is used especially if you have stop/start.

Other things to check: have you removed the silica bag of doom in the coolant header tank? If this splits, it will get into the heater matrix and stop it working (it will also get into the engine block too, so that will need urgently addressing).

Thanks. It’s a mk3.5 2020 1.5 TSI on 155k. Car sits at 90 no problem and should have the post 2018 waterpump. It doesn’t have the silica bag either. Car is saying 4 degrees this morning which is in line with phone temps.

I’m leaning more and more towards a blocked heater core. It’s going in for removal and flushing on Wednesday so I’ll know more then.

many thanks.

EDIT, sorry i see probe used.

It is the temperature in the car that matters, so use a thermometer and see what it is actually at.

At the centre of car, not at the vents.

Set to 18 *oC and see if it gets there.

Set at 25*oC and see if that is ever reached.

Getting the interior when the ambient air (outside) & the inside is 4*oC and up to 25*oC does need quite a bit of heat being generated.

PS

What actual heat do you want inside the car?

@ 20*oC i would be down to vest and pants.

(-4*oC this morning in an EV. 25 *oC to deice and clear inside took about 3 minutes.

just one of the bonuses of them.)

Edited 14 November, 202514 Nov by Evolution13

Unlikely but possibly the thermostat is failed/stuck partially opened, all VAG cars display a constant 90C between ~ 75C and 107C, in traffic, the temperature will tend to rise but on the move might be down to around 75C ish even though still reading 90C which might affect the heatinhg system.

On 14/11/2025 at 11:08, Evolution13 said:

EDIT, sorry i see probe used.

It is the temperature in the car that matters, so use a thermometer and see what it is actually at.

At the centre of car, not at the vents.

Set to 18 *oC and see if it gets there.

Set at 25*oC and see if that is ever reached.

Getting the interior when the ambient air (outside) & the inside is 4*oC and up to 25*oC does need quite a bit of heat being generated.

PS

What actual heat do you want inside the car?

@ 20*oC i would be down to vest and pants.

(-4*oC this morning in an EV. 25 *oC to deice and clear inside took about 3 minutes.

just one of the bonuses of them.)

Hi thanks. I’ve purchased a thermometer to use inside the car and the temp I set matches the temp on the thermometer. It’s 9 degrees outside and I set it for 25 inside and it said 24.8 on the thermometer. Reduce it to 22 and it goes to 22.6. So I think it’s actually working correctly. The acid test will be on a cold morning, we will see how quickly it attains the set temperature. And you’re correct. At 20 degrees setting the sweat is rolling down my back.

I was in my wife’s BMW 1 series last night and when it was set to max temp it was absolutely roasting out the vents but it doesn’t have climate control. Just standard heating.

Okay. Update time. It was 0 degrees this morning. The car took ages to reach a set temperature of 18 degrees. Now it’s 6 degrees outside it’s happily maintaining a set temperature of 21 degrees.

So it’s not working properly at very cold temps but seems to be okay when the temperature rises to above 5 degrees.

I would suggest getting the air conditioning gas checked in that case. It should be done every 2-3 years and also ask for some UV die to be put into the system to detect leaks. Get the Schrader valves changed too (high side and low side)

2 minutes ago, varaderoguy said:

I would suggest getting the air conditioning gas checked in that case. It should be done every 2-3 years and also ask for some UV die to be put into the system to detect leaks. Get the Schrader valves changed too (high side and low side)

I’m not using the air con. Just the normal heating system.

8 hours ago, gm73 said:

I’m not using the air con. Just the normal heating system.

We need a proper scan in that case.....I will not comment further. It will be either the Rain, Humidity, Light sensor; the external temperature sensor, blocked heater matrix or low gas on the HVAC.

Quite possibly not the news you want to hear and I'm hoping this isn't the case but I would absolutely get the heater matrix checked. There's an inherent issue with these engines from this era where sand was not cleared from engine casting meaning contamination in the engine block. One of the primary signs is the heater matrix get's blocked.

Skoda have just had my car (Octavia VRS Estate 2ltr for 4 months. Sand contamination in the cooling system which blocked heater matrix (only one of the internal vents in my car blew hot air the rest blew cold), blocked cooling system and ended up with blown head gasket. £7,500 later, new head gasket, replaced all cooling system parts and a load of other stuff.

The sand issue presents as small issues at first like what you've experienced or water pump fails or thermostat fails and as time goes on the issue gets progressively worse.

I'm not saying you have this problem, but just be aware that the symptoms you describe are atypical of this issue and the sooner you get to it and flush the engine the less likely it is you'll end up with a repair cost like mine.

1 hour ago, SkodaAndyVRS said:

Quite possibly not the news you want to hear and I'm hoping this isn't the case but I would absolutely get the heater matrix checked. There's an inherent issue with these engines from this era where sand was not cleared from engine casting meaning contamination in the engine block. One of the primary signs is the heater matrix get's blocked.

Skoda have just had my car (Octavia VRS Estate 2ltr for 4 months. Sand contamination in the cooling system which blocked heater matrix (only one of the internal vents in my car blew hot air the rest blew cold), blocked cooling system and ended up with blown head gasket. £7,500 later, new head gasket, replaced all cooling system parts and a load of other stuff.

The sand issue presents as small issues at first like what you've experienced or water pump fails or thermostat fails and as time goes on the issue gets progressively worse.

I'm not saying you have this problem, but just be aware that the symptoms you describe are atypical of this issue and the sooner you get to it and flush the engine the less likely it is you'll end up with a repair cost like mine.

Hi. What mileage was your car on when this issue presented itself?

Mine is on 155k now so I’d imagine the issue you experienced would surely have raised its ugly head at some point by now?

It’s going into the garage on Wednesday to have the matrix inspected and either flushed or replaced if necessary.

The strange thing is the car does reach and maintain a set temperature when the outside temperature is at 3/4 degrees or more. This morning for example it was -1 and it only really started to properly heat the cabin when the temperature rose to 3.5 degrees. It can actually get too warm after that.

There’s no issue with overheating or evidence of water pump issues. No loss of coolant etc.

***UPDATE***

It was a weak water pump causing the issues. So I’ve had it replaced and a new matrix as well as it was a bit clogged too. Full coolant flush and replacement as well.

Heaters are piping hot now thank goodness especially since the temperature dipped.

Is that a electric pump and not the main belt driven coolant pump?

32 minutes ago, Johngerard said:

Is that a electric pump and not the main belt driven coolant pump?

This one. The one driven off the cam with the solenoid and thermostat built in.

Wonder how does that belt driven pump get weak, maybe plastic impeller breaking up although I thought they had cured that problem years ago. Vag engines for years now I think incorporate two thermostats to promote quick warm up like shown below, don't know what the symphtoms are when either one fails as they open at different temperatures.

The matrix

39 minutes ago, Johngerard said:

Wonder how does that belt driven pump get weak, maybe plastic impeller breaking up although I thought they had cured that problem years ago. Vag engines for years now I think incorporate two thermostats to promote quick warm up like shown below, don't know what the symphtoms are when either one fails as they open at different temperatures.

the matrix was changed which improved things a bit heat wise but when load was reduced the air turned cold even at the highest setting. Water pump was then changed and everything was sorted.