Hello all, any Webasto experts here? 

 

This issue started about two months ago. One morning I noticed aux heater didn't start. I checked with OBD and have the fault 00006 - Heater locked: Excessive temperature

 

I unlocked it via OBD, and it worked the whole day normally, used several hours just to test it out. Then next morning, same fault.

 

Took to service, they ran tests couple of hours and didn't get the error, and measured temperatures seemed more or less normal, but the RPM of some webasto fan goes audibly a little woo-woo-woo and measured value also sails +- 200 rpm but shouldn't cause overheating according to them.

 

They suggested changing some control unit and it will cost like 1200+ and I'm very unwilling to do that for false overtemperature

 

I have ran more testing for a few days, and if in the morning I start the car, let it run for some time before starting heater, it works without issues and can run full hour. And I can do multiple subsequent heater starts without fault.

 

Then one morning I cold started it, I heard it start, runs for ~a minute, and then trips and locks before it starts blowing any air.

 

Some ideas:

- Mornings are cold

- Mornings are humid

- Running engine and fluids for few minutes gets rid of cold and humid?

- Running engine restores battery voltage? Although in one test, I started the car, and immediately after started heater, and it tripped.

 

Would appreciate any tips on saving money! 

Cannot edit anymore, but this part

Quote

Then one morning I cold started it, I heard it start, runs for ~a minute, and then trips and locks before it starts blowing any air.

got me wondering if it can be a real overtemp fault, let's say if for some reason some blower fails to start because of cold/humid/low battery, can it go to real overtemperature so fast? Otherwise I'm suspecting some measurement probe error or something

I would recommend you search in your area for a Webasto specialist centre, which for sure are more experienced in the heaters than not  Skoda dealer.

If I see my neighbour, which works in such place, I will ask him if he has come across this, what could be.

To start, do not change that control unit! Which is the entire heater.

 

Have you ever had the unit serviced? If not, it is worth doing this and replacing the thermo sensor that is built into the unit.

 

All parts are serviceable so it’s not a problem.

 

The service kit includes all new heater core as there is a chance your core coolant channels are partially blocked.

 

Have you got a full copy of the fault codes with the snap shot data that is collects, this will tell us pump rpm, temperature of coolant and of the webasto and such.

2 hours ago, ApertureS said:

To start, do not change that control unit! Which is the entire heater.

 

Have you ever had the unit serviced? If not, it is worth doing this and replacing the thermo sensor that is built into the unit.

 

All parts are serviceable so it’s not a problem.

 

The service kit includes all new heater core as there is a chance your core coolant channels are partially blocked.

 

Have you got a full copy of the fault codes with the snap shot data that is collects, this will tell us pump rpm, temperature of coolant and of the webasto and such.

Thanks for input!

 

I was wondering based on the price that there cannot be much more left to fix if the control unit change is so expensive

 

I have never had any dedicated service for it. Replacing thermo sensor sounds like something that I would like to try because I was suspecting false reading.

 

What is the service kit you're referring to? I would like to learn more about it if you have some link or part numbers or something.

 

The diagnostics service I had gave me copy of measurements they did. It's in language I don't understand but I'll try to translate interesting parts:

00006 - Heater locked: Excessive temperature
    B138F 4B [00001000] - Overtemperature

    Freeze frame:
        ...

        Parking heater: operating mode: After-run - burnout
        92
        Coolant temperature of parking heater: 46 C
        Coolant temperature: 15 C
        Engine running status: not recognized
        Fuel level: 40 L
        Heating performance: 5000 W

 

In other measurements, parking heater coolant temperature went to max 88 C, engine coolant temperature being 10-20 C lower at different points.

Battery voltage 11,3 V - 12,1 V, circulation pump voltage 14 V. (They changed the circulation pump so it was new in this test, they offered to change the old back since it wasn't the culprit)

 

Today was a 10 degrees colder temperature outside compared to previous tests, and I had to keep engine running over 10 minutes before webasto agreed to stay on (did a few unlocks and retries). 

 

Weird this inverse correlation between outside temperature and webasto overtemperature fault. Why does webasto need warmer coolant (or warmer something) in order to avoid overtemperature fault? I don't know enough of cars to explain this...

6 hours ago, Stuom said:

Today was a 10 degrees colder temperature outside compared to previous tests, and I had to keep engine running over 10 minutes before webasto agreed to stay on (did a few unlocks and retries). 

 

Weird this inverse correlation between outside temperature and webasto overtemperature fault. Why does webasto need warmer coolant (or warmer something) in order to avoid overtemperature fault? I don't know enough of cars to explain this...

 

It is firing harder to heat cold coolant than warm coolant. Whatever the issue is, maybe it is only present when it is running at higher loads. 

 

Sounds to me like the heat exchange is not as efficent as it should be due to a coolant flow restriction or build up on the combustion side causing it to trip out on high temp.

9 hours ago, SuperbTWM said:

 

It is firing harder to heat cold coolant than warm coolant. Whatever the issue is, maybe it is only present when it is running at higher loads. 

 

Sounds to me like the heat exchange is not as efficent as it should be due to a coolant flow restriction or build up on the combustion side causing it to trip out on high temp.

Thanks for answer!

 

I suspect it's not related to coolant flow, because the service I took it replaced the pump and checked coolant flow (because they assumed it's the problem).

 

The overtemperature fault also comes after webasto has been running only around 30 seconds, can this be enough time to cause a real overtemperature if combustion side is blocked?

 

I promised to call the service and answer to their offer of replacing the control unit, so basically I'm looking for ideas what to suggest them instead of replacing the control unit. But it seems the symptoms are not clear enough so I might not get out of this without spending all my money  

I asked my neighbour, and he said this type of error usually is a coolant circulation problem, either blockage on the pump side or in the heater side, or a defective pump and therefore coolant remains "trapped" in the temp reading area and quickly overheats.

On the other hand, sounds like is not the issue if you claim the pump and circuit were verified (how?).

I am also unsure as why you would have so large difference in the reported coolant temp listed 46 and 15.

I would really look in booking in some other place to have a second opinion.

can you post the entire log for the diagnosis?

Really appreciate @leolito that you cared to bother your neighbour for me!

 

Their first hunch was that the issue is the pump so they changed it, and they told they checked that it works. I don't know more details about that.

 

I copied the timestamped measurements here (these are made with the new pump after I brought the car back):

  

11:28
Measurement                      Value
Engine: coolant temp             72
Webasto: coolant temp            55
Circulation pump:                OFF

11:31
Measurement                      Value
Engine: coolant temp             59
Webasto: coolant temp            62
Circulation pump:                ON

11:32
Measurement                      Value
Engine: coolant temp             59
Webasto: coolant temp            59
Circulation pump:                ON

11:42
Measurement                      Value
Engine: coolant temp             59
Webasto: coolant temp            85
Circulation pump:                ON

11:43
Measurement                      Value
Engine: coolant temp             59
Webasto: coolant temp            88
Circulation pump:                ON

11:44
Measurement                      Value
Engine: coolant temp             50
Webasto: coolant temp            59
Circulation pump:                ON
(here they mentioned that it's normal that webasto sometimes turns off to save fuel)

11:56
Measurement                      Value
Engine: coolant temp             60
Webasto: coolant temp            80
Circulation pump:                ON

12:02
Measurement                      Value
Engine: coolant temp             68
Webasto: coolant temp            87
Circulation pump:                ON

12:15
Measurement                      Value
Engine: coolant temp             51
Webasto: coolant temp            71
Combustion air blower:           6308 (RPM)

In addition there are some graphs that measure combustion blower RPM which show that the value sails between ~6350-6275 RPM in 10-second measurement.

 

From another forum I found a post that has exactly the same error codes as I do, and similar temperature difference in the freeze frame. Their cause was temperature sensor.

 

I think the main suspects are now temperature sensor and/or blockage in the combustion side?

 

Edited 4 November, 20241 yr by Stuom

Well, we are both technically minded and this is an interesting case, which I am sure he would have jumped at it .... that is why I recommended if you have in the vicinity a specialized Webasto service to go see them, you can get a second opinion and more in depth ....

My neighbours' service gets many VAG cars redirected from the official dealer when they cannot fix it or just raise their hands ....

 

While the Superb is my first car with built-in aux heater, I've had at least another 4 heaters which I've bought second hand, made the circuit for them, tested them and ...believe it or not I've sold the cars - and afterwards the heaters - before mounting them ... now I've got somewhere still the last one, a petrol one, which if i get my hands on a Landie I fancy I will convert it to Diesel and perhaps finally I will succed .... 🤪

 

The temp sensor could very well be the culprit. You could have electrical circuit hit-and-miss failure, for example corrosion on the connecting plug which give untable connection, same could happen to the 12V to the circulation pump .... I think for the temp sensor you need again to remove the heater.

I observed this error several times when coolant temperature was high like 50c. Whenever coolant temperature exceeds the limits(I am not sure  what limit is) then it stops working.

Also I was having similar errors last year it stoped working I had to replace webasto pump then heater core. 

Edited 6 November, 20241 yr by candasm

Hey.

Did you find any solutions? I would like to say that I have exactly the same problem.

@Stuom

Hey,

Can you answer me which part was the problem this time and how did you managed to replace it?

Hi, sorry I don't have an answer, mine is still broken because I'm lazy... I'll report here if I ever get to fix it

Skoda Superb trouble code B138F4B

Can someone help me how to solve this problem?

Hi, I had the same fault code and, after some diagnostics, found that I had a faulty overheat sensor.

The one with red wires.

On 04/09/2025 at 14:48, Rehamang said:

Skoda Superb trouble code B138F4B

Can someone help me how to solve this problem?

Check

On 04/09/2025 at 14:48, Rehamang said:

Skoda Superb trouble code B138F4B

Can someone help me how to solve this problem?

Check overheat sensor reistance

On 18/11/2025 at 19:49, Mevlan said:

Hi, I had the same fault code and, after some diagnostics, found that I had a faulty overheat sensor.

The one with red wires.

Did that solve the problem?

On 18/11/2025 at 19:51, Mevlan said:

Check

Check overheat sensor reistance

Can you point to me where can I find the whole document?

My octavia have the same fault code, and after looking at the data it shows "overtemp" on outside temperature.

@Mevlan @PronixZero

Sorry for replying that late. I had exactly the same problem as @Stuom . I replaced the temperature sensor (5Q0898047 part number for mine) and now the Webasto works perfectly again. So yes, thanks Mevlan — the temperature sensor was indeed the issue.

I replaced mine as well and it solved the issue.

On 07/12/2025 at 11:07, Rehamang said:

@Mevlan @PronixZero

Sorry for replying that late. I had exactly the same problem as @Stuom . I replaced the temperature sensor (5Q0898047 part number for mine) and now the Webasto works perfectly again. So yes, thanks Mevlan — the temperature sensor was indeed the issue.

After this procedure, is it necessary to bleed the system?

I didn’t bleed it properly afterward. I clamped/blocked the hoses, so coolant loss was nearly zero. After the replacement, I topped up the coolant with less than 0.5 L. What I did was run the pump manually via OBD, so the air bubbles were gone when I initially started the system.

Could you share how I can run it manually via OBD.

Edited 9 February9 Feb by NDW89

You need to have a bidirectional capable obd reader, like VCDS, ds150e or OBDeleven. In the app first you need to conenct to the climate control module, there will be an option for bidirectional testing, it's the menu where you can turn on/off some funtions of the car based on the module you are in, in this example (fans, pumps, etc....)