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Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)

Piter.Krr 45186 50
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How do I fix an F27 error on a Vaillant ecoTEC Plus VC PL306/3-5 boiler that will not ignite even after cleaning the electrodes and drying the board?

F27 on this boiler usually points to the control board rather than the flame sensor itself, and if a reset, drying the PCB, and cleaning the ignition electrode do not help, the most common fix is to replace the board’s aged electrolytic capacitors [#18476980][#18477730] Several users restored operation by replacing the two small 470 µF/35 V capacitors on the board, often identified as C5/C30 and C9/C29, and one also reported success after cleaning the X20 connector and checking the relay contacts [#18481506][#20765203][#21781800][#18477730] Clean the electrode to a metallic sheen and inspect the X20 connector for moisture, but if the error still returns, the board itself may need replacement [#18476980][#18477730]
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  • #1 18326756
    Piter.Krr
    Level 9  
    Posts: 32
    Rate: 12
    Hi.

    Yesterday (Saturday) evening, the stove displayed error F27 and does not work.
    The manual says: meaning - foreign light source, cause - faulty flame sensor.

    I honestly didn't find something like a flame sensor (but maybe because I'm rare in this topic).

    What I have done so far:
    1) I dried the plate with a dryer - because I read that the reason could be moisture and that it helped someone in England :)
    2) I cleaned the ignition electrodes
    3) turned off/on

    Furnace data:
    Vaillant Eco TEC Plus VC PL306/3-5

    Motherboard data:
    sticker - 0020035421
    overprint - TM-AC1 V132_C1 0020028453 TL-AC PM-03

    Help, it would be great to start the heating today.
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  • #2 18327522
    waldi644
    Level 15  
    Posts: 201
    Help: 12
    Rate: 39
    Emergency shutdown: flame simulation - Wet electronics, defective electronics (flame control), leaking gas solenoid valve
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  • #3 18327588
    Piter.Krr
    Level 9  
    Posts: 32
    Rate: 12
    Thanks for showing interest.
    Wet electronics - that's why I dried it
    Damaged electronic system - I would love to find out what the most common causes were and proceeded to action :)
    leaky valve - no smell of gas, if that is any indicator
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  • #4 18328105
    ls_77
    Level 38  
    Posts: 2439
    Help: 500
    Rate: 943
    Did you reset the boiler by accepting the error with a button with a crossed out flame?
    Did the boiler turn on then?
    When does F.27 error pop up? During the operation of the burner, how does the flame burn or after the end of the operation of the burner - after the flame goes out?
    The fact that it signals a "foreign light source" means that the electronics still reads the ionization current from the electrodes after the signal, turn off the gas valve and burner.
    Maybe you have a lot of impurities in the combustion chamber that get very hot and after turning off the gas valve and the burner, the electronics reads that there is still a flame on the burner? Maybe the gas valve does not close properly and still lets in small amounts of gas and the flame is still burning after the signal of gas closing? Can you see any of the above through the burner inspection window?
    Or maybe the motherboard?
  • #5 18328119
    Piter.Krr
    Level 9  
    Posts: 32
    Rate: 12
    ls_77 wrote:
    Did you reset the boiler by accepting the error with a button with a crossed out flame?
    Did the boiler turn on then?

    answer That's what I did, then the boiler turned on again, trying to work (it takes maybe 10 seconds) and again the error and silence

    ls_77 wrote:
    When does F.27 error pop up? During the operation of the burner, how does the flame burn or after the end of the operation of the burner - after the flame goes out?

    answer It pops up every time you start it, that is, it doesn't even ignite.

    ls_77 wrote:
    Maybe you have a lot of impurities in the combustion chamber that get very hot and after turning off the gas valve and the burner, the electronics reads that there is still a flame on the burner? Maybe the gas valve does not close properly and still lets in small amounts of gas and the flame is still burning after the signal of gas closing? Can you see any of the above through the burner inspection window?

    answer I think it's out because the flame hasn't been on since Saturday night.

    ls_77 wrote:
    Or maybe the motherboard?

    answer Right, but what about her? Maybe there are some typical faults, some parts that need to be replaced. I have some electronic components at home, but little knowledge.
  • #6 18328377
    ls_77
    Level 38  
    Posts: 2439
    Help: 500
    Rate: 943
    This is not a common and repetitive error. I've never met him before.
  • #7 18328892
    Piter.Krr
    Level 9  
    Posts: 32
    Rate: 12
    What else did I do:
    I checked for gas :)
    - I checked that there is no voltage to the gas solenoid valve
    - I checked the gas solenoid valve giving 24V voltage, the solenoid valve works (of course with the gas valve turned off in front of the stove)
    - capacitor C702 is charged with 110V
    - capacitor C3 is charged with 313V

    Please help.
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  • #8 18473165
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    Hi. I have exactly the same problem as you, except that when my boiler went into failure for the first time, it displayed error F.75. Then I reset the boiler. From then on, only error F. 27 appears. However, I have the impression that the pump makes a different noise than during normal operation.
    Have you solved your problem? If so, tell me how. Thank you.
  • #9 18476980
    VPS
    Level 25  
    Posts: 547
    Help: 87
    Rate: 229
    With error F.27 in 95% the electronic board needs to be replaced. The remaining 5% is the electrode to be replaced.
    Perhaps the x20 connector on the electronic board is wet.
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5) Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
  • #10 18477622
    kredar
    Level 1  
    Posts: 1
    Rate: 4
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
    Hello
    I have the same problem. Pressing the button with the crossed out flame several dozen times helped until today. Unfortunately, I can't run the furnace this way at the moment.
  • #11 18477730
    ROWE
    Level 34  
    Posts: 1987
    Help: 239
    Rate: 529
    The electrodes in photo # 9 should be cleaned to a metallic sheen, that's the basis.
    You can replace the 3 largest electrolytic capacitors on the board, it helps. There are descriptions on the forum.
    It is also worth checking the resistance of the NO and NC contacts in the relays.
    If that doesn't work then the board needs to be replaced.
  • #12 18481506
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    It worked!!! The boiler has started!!!
    But one thing at a time: the X20 connector was dry, I cleaned the electrode to a metallic sheen - it didn't help, so I thought it was a disc, I even found one in a nearby Vaillant store for PLN 1022, but I wouldn't be myself if, using your glad he didn't try to fix it. So I bought 3 electrolytic capacitors for PLN 7.80. I replaced the two smaller ones out of the three and it was "shot 10". The boiler has been heating for 3 hours. But I'm happy :)
    To clarify the information: the repair concerned the Vaillant VC PL 186/3-5 boiler, on the board no. 0020035421 I replaced 2 twin capacitors 470 uF 35V.
    Thank you all for your help!!!
  • #13 18481598
    ROWE
    Level 34  
    Posts: 1987
    Help: 239
    Rate: 529
    Well, I am full of admiration, as for an amateur, you have 1000 zlotys ahead! This applies to most old discs and also error F75, 61. Practice makes perfect.
  • #14 18530213
    Dbdavi
    Level 1  
    Posts: 1
    Rate: 1
    I have the same thing, do these capacitors need to be entitled somehow or can they be replaced non-invasively?
  • #15 18648956
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    I don't quite understand the question. What does "entitle capacitors" mean? You have to desolder old capacitors and solder new ones in their place. This means that you have to remove the board and use at least a soldering iron, replace the capacitors and reinstall the board. I don't know if these activities can be considered invasive or not? The disassembly and assembly of the board itself is not difficult, when replacing capacitors you need to have at least minimal skills in using a soldering iron. I'm not an expert, but I did. The boiler is still working.
  • #16 19264726
    cynk23
    Level 1  
    Posts: 1
    Hello. Is five still going? I have a similar problem for a few days, first there was an error F62 after a reset, five went for several days and now displays F27 again reset five goes one day and again blonde F27 this time Mosze reset a few times and will surprise. What model do you have oven. Mine is eco tec plus 186
  • #17 19282777
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    Good morning, yes the boiler is still working. I confirm that in my case, for the error F.27 and the lack of boiler operation, the replacement of capacitors was an effective and effective repair. Yes, after that repair, the boiler once again refused to cooperate with me, but the problem was the worn out circulation pump. Then the F.75 error appeared (but it's to be checked if necessary, because I'm giving it off the top of my head). Yes, I have a Vaillant VC-PL 186/3-5 boiler.
  • #18 19303367
    toretto25
    Level 2  
    Posts: 4
    Rate: 2
    Pepe, can you mark on the drawing which capacitors you replaced? I have the same problem.
  • #20 19303404
    toretto25
    Level 2  
    Posts: 4
    Rate: 2
    Thank you very much, I will delegate it to someone else. I'll let you know how the replacement went. For now, I have a new one from a service friend.

    Added after 1 [minutes]:

    So blue and the two black ones?

    Added after 40 [minutes]:

    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
    Pepe or start with two twins and possibly the big one? I am a complete layman
  • #21 19304123
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    Yes, two black 470uF and a blue 100uF.
    I don't know which one was faulty because I replaced all three at once. I recommend you too, the cost is small, and they get damaged over time, so there is a probability that even if you hit the one that was damaged, the next ones will be damaged in a moment.
  • #22 19306272
    toretto25
    Level 2  
    Posts: 4
    Rate: 2
    Pepe, thank you, one of the twin small ones listed two, the 400v one stayed because it is working. Thanks again for your help.
  • #23 19407220
    lechooo93
    Level 9  
    Posts: 23
    Rate: 14
    I had the same problem myself. The capacitors were in good condition, so I cleaned the contacts with a contact cleaner and it helped - specifically the contacts from the electrode (ground + this pin). For future generations, I recommend giving it a try ;)
  • #24 19698522
    mmaciaszek
    Level 2  
    Posts: 3
    Does anyone know the dimensions of this large 100uF 400V capacitor? Does it have long thin pins to solder or short thick ones?
  • #25 19699977
    rnb_bolii
    Level 39  
    Posts: 4164
    Help: 634
    Rate: 1172
    Each new capacitor has long "legs. So what's the problem ... I can put it in.
  • #26 19700169
    mmaciaszek
    Level 2  
    Posts: 3
    Some have these short and thick legs Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5). They also have different diameters and heights. I would like to have this data to buy the right one without disassembling the board first.
  • #27 19701029
    pepe__75
    Level 11  
    Posts: 19
    Rate: 46
    If I remember correctly they were thin long
  • #28 19906234
    VPS
    Level 25  
    Posts: 547
    Help: 87
    Rate: 229
    original desoldered
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
  • #29 20509748
    szymanek732
    Level 2  
    Posts: 2
    I dried the plate and started from where the moisture came from
  • #30 20554551
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    I'm struggling to get the plate out of the boiler. Unfortunately, cleaning the electrode and drying x20 did nothing. Does anyone know if the c25 / c27 connector (yellow-green cables) is pulled out or should it be desoldered?
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
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Topic summary

✨ The Vaillant EcoTEC Plus VC PL306/3-5 boiler displayed error F27, indicating a flame sensor fault or detection of a foreign light source, preventing ignition. Initial troubleshooting included drying the electronics board, cleaning ignition electrodes, and resetting the boiler, but the error persisted. The F27 error often relates to faulty flame detection circuitry or moisture affecting the electronic board, particularly the X20 connector. Common solutions involve cleaning electrodes to a metallic sheen and replacing electrolytic capacitors on the motherboard, especially two twin 470 µF 35V capacitors and a 100 µF capacitor, which degrade over time and cause flame detection failures. Replacement requires desoldering and soldering skills. Some users reported success by cleaning contacts with contact cleaner or ensuring dry connectors. The gas solenoid valve and gas supply were verified functional. The error is uncommon and may also be linked to worn circulation pumps causing secondary errors like F75. Capacitor replacement is a cost-effective repair compared to full board replacement. Detailed capacitor locations (C5, C9) and photos were shared. The discussion also covered capacitor physical characteristics and connector removal techniques. Overall, the F27 error on Vaillant EcoTEC Plus boilers is primarily resolved by capacitor replacement and thorough cleaning of flame detection components and connectors.
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FAQ

TL;DR: 95 % of Vaillant F.27 faults trace back to the control board; “replace the board or the electrode” [Elektroda, VPS, post #18476980] Swapping two capacitors (PLN 7.80) revived one boiler that a shop quoted PLN 1022 to fix [Elektroda, pepe__75, post #18481506]

Why it matters: A 10-minute DIY fix can restore heat and save over PLN 1000.

Quick Facts

What exactly does the Vaillant F.27 error code mean?

F.27 signals that the PCB still “sees” a flame when the gas valve should be closed. The firmware interprets this as a foreign light source or flame simulation and shuts the boiler down for safety [Elektroda, Piter.Krr, #18326756; Elektroda, ls_77, #18328105].

Why does my boiler throw F.27 before it even ignites?

If the ionisation circuit is noisy, the board reads residual voltage as a flame as soon as it starts its test cycle. Because no real ignition occurs, the fault appears within 10 s of each reset [Elektroda, Piter.Krr, post #18328119]

Which part fails in most F.27 cases?

A service engineer states that 95 % of F.27 incidents involve the electronic control board, often due to degraded capacitors; only 5 % involve the flame electrode itself [Elektroda, VPS, post #18476980]

Which capacitors should I replace first?

Replace the two electrolytic capacitors marked C5 and C9 (220–470 µF, 35 V). Users fixed multiple boilers by swapping just those parts [Elektroda, Adewag, #20557245; Elektroda, pepe__75, #18481506].

How do I replace them?

  1. Isolate power and remove the front panel.
  2. Unclip the PCB, desolder C5 and C9, fit new 35 V capacitors observing polarity.
  3. Re-install the board and reset the boiler. Many users restored operation within 30 minutes [Elektroda, sosna2, post #20765203]

Do capacitors need “entitlement” or special coding?

No. Standard radial electrolytics with the same capacitance and voltage rating work. One user desoldered old parts and soldered replacements without any programming or coding [Elektroda, pepe__75, post #18648956]

My large 100 µF/400 V capacitor has three legs—normal?

Yes. Some original boards use a dual-ground, three-pin layout. Only capacitance (100 µF) and voltage (400 V) matter when buying a replacement [Elektroda, Adewag, post #20554823]

What tools do I need for the job?

You need a 40 W soldering iron, desoldering braid or pump, and basic pliers. No specialised Vaillant tools are required [Elektroda, pepe__75, post #18648956]

Can moisture alone trigger F.27?

Yes. Wet electronics or a damp X20 connector can mimic a flame signal. Drying the board sometimes clears the error temporarily [Elektroda, VPS, #18476980; Elektroda, Piter.Krr, #18326756].

What rare edge-case faults also cause F.27?

Leaking gas valves, relay contact failure, or combustion-chamber hotspots can keep a real flame burning after shutdown, confusing the sensor [Elektroda, waldi644, post #18327522]

Is F.27 related to F.75 or F.62?

They are different checks. However, users have seen F.75 (pump pressure) or F.62 (gas valve closure) immediately before or after F.27 when the PCB is weak, so repairing the board often clears multiple codes [Elektroda, pepe__75, #19282777; Elektroda, cynk23, #19264726].

How do I reset the boiler after repair?

Press the button with a crossed-out flame once. The display clears the code and restarts the ignition sequence [Elektroda, ls_77, post #18328105]

How do I remove the yellow-green C25/C27 ground plug?

Pull the cable and black plug together; a small latch holds it. Pliers help if space is tight [Elektroda, Adewag, post #20554601]

Will cleaning contacts ever fix F.27 without soldering?

Sometimes. One user cured the fault by spraying contact cleaner on the electrode and ground connections after confirming capacitors were healthy [Elektroda, lechooo93, post #19407220]
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