Hi, i have a Skoda octavia 2, 1.2 TSI, CBZB and I had problems with the old thermostat: the engine was warming up very slowly and barely reached 50°C, and the radiator fan would turn on even when the temperature didn’t go above 50–60°C.

I replaced it with a Topran thermostat, but now the symptoms are similar. The engine doesn’t go above 70°C, it reaches that temperature very slowly after long driving, and if I stop accelerating, the temperature drops.

The radiator fan does not turn on, but the hose going to the radiator is hot. Is it possible that the new thermostat is also defective, or could there be another cause?

Wonder if you have an airlock somewhere after changing the stat. Some require vcds or a vaccum fill to bleed properly. Not sure on your CBZB engine. Try and squeeze top rad hose again and again and look for bubbles in coolant tank or see if it drops. Do the heaters work in car?

Alasdair

When I replaced the gasket between the oil filter housing and the engine on my Fabia 1.2 TSI CBZA (same as CBZB), both the oil and the coolant had to be drained. To remove any air pockets, you should let the engine idle with the coolant reservoir cap open, while keeping the heater on full blast. This worked for me, and I haven’t had any issues even after two years of really hard use.

On 20/01/2026 at 12:34, Alasdair1 said:

Wonder if you have an airlock somewhere after changing the stat. Some require vcds or a vaccum fill to bleed properly. Not sure on your CBZB engine. Try and squeeze top rad hose again and again and look for bubbles in coolant tank or see if it drops. Do the heaters work in car?

Alasdair

The heater works, i squeezed the hose and the water from the coolant tank was moving, that's all, that means i have air in the system? But even if i have air in the system, the thermostat is supposed to let coolant through the Radiator Hose even if the temperature is not even above 50??

On 20/01/2026 at 12:59, Jack25 said:

When I replaced the gasket between the oil filter housing and the engine on my Fabia 1.2 TSI CBZA (same as CBZB), both the oil and the coolant had to be drained. To remove any air pockets, you should let the engine idle with the coolant reservoir cap open, while keeping the heater on full blast. This worked for me, and I haven’t had any issues even after two years of really hard use.

I did that a few times. First time was when i changed the new thermostat.

Sorry if its an obvious question but did the Topran one come with a thermostat actuallyfitted. I see them online but its the casing only unlike the Febi one that has thermostat included/fitted. Also read that the temp sender in thermostat on topran can cause problems sometimes with turning on rad fans.

Alasdair

If @klobb still has the old stat, see (if you can) if its partially open, if not, put it in a pot and see what temperature it begins to open, should be ~ 84/88C

2 hours ago, Johngerard said:

If @klobb still has the old stat, see (if you can) if its partially open, if not, put it in a pot and see what temperature it begins to open, should be ~ 84/88C

Yes, i did that test, the old thermostat does nothing when i sinked it in boiling water.

I also removed it from the housing and the spring is expanded at maximum i tried hot water/ cold water and no changes.

My question is, if the top hose radiator should get hot after few minutes after starting the engine or when it reaches 90 degrees.

Edited 23 January23 Jan by klobb

26 minutes ago, klobb said:

Yes, i did that test, the old thermostat does nothing when i sinked it in boiling water.

I also removed it from the housing and the spring is expanded at maximum i tried hot water/ cold water and no changes.

My question is, if the top hose radiator should get hot after few minutes after starting the engine or when it reaches 90 degrees.

Strange how the new stat is aparently "open" with cold/cool engine coolant temperature.

The (any) stat shouldn't begin to open at say 84C ish and go fully open at ~ 98C.

All Vag cars will display 90C between a true coolant temperature of ~ 75C and ~ 107C so your displayed temp should reach 90C well before the stat starts opening/top hose getting hot, you can see my 2019 Polo just reaches a displayed temperature of 90C when the true temperature only reaches 75C, if yours only reaches a displayed 70C, ~ 63C true temperature even while driving then it would appear that this new stat is also passing (even though one would think unlikely) or else installed incorrectly, if this is possible.

VW Polo  02092024

Coolant Temp

VCDS/Car Display

52 /52

53/55

56/58

58/62

60/65

63/69

64/72

66/74

68/78

69/82

71/84

73/86

74/88

75/89

75/90

Edited 23 January23 Jan by Johngerard

1 hour ago, Johngerard said:

Strange how the new stat is aparently "open" with cold/cool engine coolant temperature.

The (any) stat shouldn't begin to open at say 84C ish and go fully open at ~ 98C.

All Vag cars will display 90C between a true coolant temperature of ~ 75C and ~ 107C so your displayed temp should reach 90C well before the stat starts opening/top hose getting hot, you can see my 2019 Polo just reaches a displayed temperature of 90C when the true temperature only reaches 75C, if yours only reaches a displayed 70C, ~ 63C true temperature even while driving then it would appear that this new stat is also passing (even though one would think unlikely) or else installed incorrectly, if this is possible.

VW Polo  02092024

Coolant Temp

VCDS/Car Display

52 /52

53/55

56/58

58/62

60/65

63/69

64/72

66/74

68/78

69/82

71/84

73/86

74/88

75/89

75/90

I found out that the coolant temperature sensor is faulty, showing -11 degrees with 2 degrees outside. But that should not be a problem because the thermostat is 100% mechanical, right?

Maybe the new stat is deffective or something in the coolant system has jammed it open. Reckon the only way to be sure will be to remove it and check its operation. Its not unknown for any new part to be dodgy or fail very quickly

On 23/01/2026 at 13:35, klobb said:

My question is, if the top hose radiator should get hot after few minutes after starting the engine or when it reaches 90 degrees.

as radiator has only two connections top hose cannot get hot without warm water going from bottom hose and through radiator. I assume that if radiator is cold and top hose getting warm there is a premature opening of thermostat. It is unhealthy to sink thermostat in the boiling water. I would rather buy two three wahler behr vernet and compare opennings in the pot to chose the narrow one.

I replaced thermostat recently. Im driving to my work 4km without warming up. Hand goes to 90'C after 3km. While there is demand from heater core hoses never get warm so I assume big circuit remains closed.

Edited 24 January24 Jan by kokosal

On 23/01/2026 at 15:27, klobb said:

I found out that the coolant temperature sensor is faulty, showing -11 degrees with 2 degrees outside. But that should not be a problem because the thermostat is 100% mechanical, right?

Yes, and no, a faulty sensor will have no effect on the stat's opening temperature, but one would think that the coolant display should have stayed "permanently" below 50C as I presume it uses the same sensor.

If those VCDS readings were taken with the engine idling then the 95.3C Rad outlet doesn't tell a awful lot because the stat, even if working perfectly, will normally (eventually) go full open while idling and the radiator fan will cut in/out.

Replacing that sensor will tell alot if VCDS still available to you.

I looked at that VCDS incorrectly, the rad coolant outlet was apparently 225C, 95.3C might be the temperature which starts the radiator cooling fan, but if the coolant was 225C then the equivalent (saturation) pressure is > 25bar but the cooling system PRV lifts at ~ 1.5bar so don't know what's going on there.

3 hours ago, Johngerard said:

I looked at that VCDS incorrectly, the rad coolant outlet was apparently 225C, 95.3C might be the temperature which starts the radiator cooling fan, but if the coolant was 225C then the equivalent (saturation) pressure is > 25bar but the cooling system PRV lifts at ~ 1.5bar so don't know what's going on there.

Clearly, 225 deg.C isn't possible without the coolant system venting - so there is at least a sensor issue to address.

It says 2.25C, I think.

Which may be consistent with no flow through rad at that stage.

So radiator outlet temp sensor probably OK, main engine one not so good.

Edited 25 January25 Jan by Breezy_Pete

You're right re the 2.25C but the stat should still operate normally?. I'd like to know what this 95.3C "radiator output specified value coolant temperature" refers to, is it the (a) actual radiator outlet coolant temperature.

Edited 25 January25 Jan by Johngerard

My policy with repairs is to fix each evidenced problem as they become apparent.

If the overall situation doesn't improve, look for the next bit of evidence.

Quite often the first repair is sufficient, without it necessarily being clear why.

I'm not sure that the thermostat was ever misbehaving. If the engine ECU was getting unreliable temperature info, it can't control things correctly.

Edited 25 January25 Jan by Breezy_Pete

Just came across this on some VCDS website...

IDE00025 Coolant temperature 85 °C
IDE00192 Coolant temperature at radiator outlet: actual value 55.2 °C
IDE00194 Coolant temperature at radiator output: specified value 90.0 °C
ENG105382 Coolant temperature 85 °C
ENG106503 Coolant temperature at radiator outlet 85 °C

The problem was at the coolant temperature sensor, i changed it and now works good.