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[Solved] Skoda Octavia Mk1 1.6 AVU/BFQ Thermostat Only Opens at Max Heat Setting—Why?

iuliancristian 660 10
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  • #1 21581810
    iuliancristian
    Level 2  
    Hello,

    I own a Skoda Octavia Mk1, 2002, 1.6 petrol, engine code AVU, BFQ.
    While waiting in traffic last year, I noticed the temperature was over 90 degrees. I pulled over, opened the hood, and noticed the fans weren't spinning. If I turned on the air conditioning, they would spin. During the diagnosis, I noticed that the thermostat, which is controlled by the ECU, only opens with the cold/hot potentiometer at maximum heat. If the potentiometer is at minimum cold or at an intermediate position, the thermostat does not open. Basically, the coolant reaches 95 degrees, the thermostat opens, it starts flowing through the radiator, the sensor on the radiator registers the temperature increase and at around 70 - 80 degrees the radiator temperature, the fans start rotating at second speed.
    Before, it would turn on the fans at 90 degrees, with the first speed. I replaced the fans and the fan control module and the same thing happened.
    I can't use the air conditioning in such conditions.
    AI: Have you checked or replaced the coolant temperature sensor(s) on the engine? If so, did you notice any change in the thermostat or fan behavior?
    Yes
    AI: Have you scanned the car for any fault codes (using OBD diagnostics), and if so, did any codes come up related to the cooling system or sensors?
    Yes
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  • #2 21581947
    andrzej20001
    Level 43  
    Maybe the fuse 1 of the ventilator fuse on the battery is faulty. No. 3 and 6
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  • #3 21581964
    iuliancristian
    Level 2  
    Hello,

    They made a very big mistake when they implemented the ECU controlled thermostat. If it was classic, everything would be ok. The problem is with the thermostat, why does it only open with the knob at full right. If I turn on the air conditioning, the fans spin at speed 1. So it wouldn't be a fuse issue. First the thermostat opens, allowing the coolant to flow to the radiator and then the fans start. They have no reason to start if the coolant in the radiator has a temperature of 30 - 40 degrees.
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  • #4 21581993
    danielzy
    Level 27  
    On this car the thermostat is not controlled by the ECU in any way. It's a normal thermostat that has full opening at 87st as I recall and it has nothing to do with the position of the potentiometer from the heater. You may have the wrong thermostat fitted.
  • #5 21582113
    iuliancristian
    Level 2  
    Hello,

    My car has a thermostat controlled by the ECU, it doesn't have a switch on the radiator to control the fans. The thermostat has two wires that go directly to the ECU and without its command it won't open. It doesn't open even at 115 degrees. It has a paste, a wax inside and the ECU controls its heating and thus it opens.
  • #6 21582190
    danielzy
    Level 27  
    Sorry you are right , I checked and there were 2 types of thermostats , the regular one and the extended one with control. Maybe try replacing the thermostat with another piece that works.
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  • #7 21582386
    iuliancristian
    Level 2  
    Hello,

    Replaced without any effect. I'll do more research on this way of managing the thermostat.

    Added after 9 [hours] 5 [minutes]:

    Exactly what I was mentioning. It has to do with the temperature control knob.
    There are 5 factors that contribute to the degree of thermostat opening, along with ABS. Now I know where to check.
    Skoda Octavia Mk1 1.6 AVU/BFQ Thermostat Only Opens at Max Heat Setting—Why?
  • Helpful post
    #8 21583361
    wilhelmk
    Level 12  
    Replacing the thermostat makes no sense. In this engine the operating temperature reaches 115 degrees. Fellow easterners have mastered the subject:
    https://www.drive2.ru/l/450991633240949386/
    It is enough to swap two pins in the temperature knob block so that the thermostat opens at 95 degrees when set to the "cold" position
  • #9 21583550
    iuliancristian
    Level 2  
    Hello,

    Thanks for the reply and the link. I found the following in the official documentation:


    Skoda Octavia Mk1 1.6 AVU/BFQ Thermostat Only Opens at Max Heat Setting—Why?
    Diagram showing a car cooling and thermostat control system, with N147 valve and vacuum actuator labeled.

    I will check first the valve N147 and vacuum-operated actuator. Then I'll check the wires which come from the knob. If these two things are ok, I will proceed as per the link you sent.

    To recap, the thermostat doesn't open even at 115 degrees with the potentiometer set to "cold". With it set to "hot" it opens at 95 degrees. And this happened suddenly, in traffic. Otherwise the car works well, has power, no errors. But I can't use the air conditioning in such situations. Since this situation arose last year, I checked whenever I had time, in the garage, the wires in the engine compartment, under the battery, I opened the plenum chamber, under the dashboard. I opened the fuse box, checked the wires near the ECU connectors. Everything is fine, there are no bare wires.
  • #10 21584560
    iuliancristian
    Level 2  
    @wilhelmk

    Thanks for the instructions, I chose to apply the solution from the link you sent.
    Now the thermostat opens at 90 degrees, the engine block temperature reaches 95 degrees and the radiator at 70 degrees, and then the fans start. That is, everything is as it should be. Now I can use the air conditioning. Regarding heating in the cold season, I either repeat the operation with the contacts when the temperature drops outside, or I buy a powerful indoor fan heater.
    Thank you once again.
  • #11 21585233
    iuliancristian
    Level 2  
    User wilhelmk provided me with the solution.
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Topic summary

A Skoda Octavia Mk1 2002 1.6 petrol (engine codes AVU/BFQ) exhibits an issue where the ECU-controlled thermostat only opens when the heater potentiometer is set to maximum heat, preventing normal coolant flow and causing overheating in traffic. The cooling fans do not activate unless the air conditioning is on, which forces fan operation at speed 1. The thermostat is not a conventional wax-type but an ECU-controlled unit with a wax element heated electrically to regulate opening. Replacing the thermostat and fan control module did not resolve the problem. Investigation revealed that the thermostat opening degree depends on the heater temperature control knob position, which influences ECU commands. A known solution involves swapping two pins in the heater temperature knob wiring block to allow the thermostat to open at normal operating temperatures (around 90-95°C) even when the knob is set to cold. After applying this modification, the thermostat opens correctly at 90°C, coolant flows through the radiator, radiator temperature reaches about 70°C, and fans start appropriately, enabling normal air conditioning use. Additional checks include verifying the vacuum-operated valve N147 and associated wiring. This ECU-controlled thermostat system is specific to certain Skoda Octavia Mk1 variants and differs from the standard mechanical thermostat.
Summary generated by the language model.
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