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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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  • #31 20554591
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    If I understand correctly, it is removed.
    Attachments:
    • Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5) IMG_20230425_110427201.jpg (4.22 MB) You must be logged in to download this attachment.
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  • #32 20554599
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    Thank you very much. I pry the black cover from the bottom near the plate and it doesn't go up. I apologize for perhaps a naive question, but should I pull the cables from above, i.e. above the black cover - the cables should come out of the cover and it should remain, or maybe the cover comes out with the cable?
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
  • #33 20554601
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    I pulled the cable and the plug hard at the same time. There is a hole for the "tab" and it locks.
  • #34 20554628
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    I'll try it soon. So the cable came out with a black plug?
  • #35 20554682
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    Yes, with the plug.
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  • #36 20554692
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    Thank you for the info. I grabbed the pliers for the plug and it came out without a problem. Is it possible that this large 400 capacitor is 3-leg?
    Vaillant EcoTEC Plus F27 Error: Flame Sensor Issues & Troubleshooting (VC PL306/3-5)
  • #37 20554823
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    That's what you see in the photo. Twice the mass.
  • #38 20555545
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    So one with 3 legs, so a bit different than what was written before? Do you know why I have another?
  • #39 20555745
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    It doesn't matter, it's the capacitance and voltage that count.
  • #40 20556925
    Jabbi
    Level 6  
    Posts: 36
    Rate: 3
    Successful boiler fixed error disappeared. Cost PLN 2.70 for two smaller twin capacitors that had to be desoldered by inserting new ones. The big one turned out to be functional and stayed in place. Thanks to everyone who posted in this thread, it was worth a try.
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  • #41 20556931
    rnb_bolii
    Level 39  
    Posts: 4164
    Help: 634
    Rate: 1172
    What capacitors? Can you pass?
    C9 and C5?
  • #42 20557245
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    Capacitors C9 and C5 are the main cause of faults on this board, 220 micro at 35 V.
  • #43 20566979
    adobek
    Level 1  
    Posts: 1
    Rate: 1
    >>20556925
    Thanks. Just turned on my boiler after replacing two 470 uF capacitors. Works.
  • #44 20574975
    wingerman
    Level 11  
    Posts: 33
    Rate: 3
    These 420uf microfarad capacitors are for what voltage?
  • #45 20574995
    Adewag
    Level 21  
    Posts: 1080
    Help: 36
    Rate: 249
    Na 35v.
  • #46 20575880
    wingerman
    Level 11  
    Posts: 33
    Rate: 3
    Thank you very much my friend.
  • #47 20765203
    sosna2
    Level 2  
    Posts: 4
    Rate: 1

    Hello cordially :) I would like to thank the esteemed forum members very much for their help. Vaillant ecoTEC plus PL 246/3-5 R3 boiler, as in the topic - F.27 error popped up, resetting several times did not help, the boiler would start up but after trying to fire the flame immediately the error popped up. Cleaning the electrode did not help. I know nothing about electronics, but emboldened by esteemed colleagues I disassembled the plastics and got to the motherboard, purchased 2 small capacitors located on the board at the bottom left (in my case 470 uF, 35V), soldered them without disconnecting the cables - just the board itself I released from the hooks and lifted up so that there was access from below to approach the soldering iron (well, the wife helped) and the boiler came to life!!!! once again many thanks :) )))
    ps. cost of capacitors 2 zl.
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  • #48 21433697
    EkoGrzew
    Level 20  
    Posts: 431
    Help: 28
    Rate: 231
    Hello, I am reviving the topic.
    In my case at the service, the boiler after opening the chamber has not been cleaned for probably 8 years...
    so first cleaning and maintenance. . then checking the coils on the gas valve. Tomorrow I'll check the capacitors suggested here. I'd have to check the connections as it's clearly a lack of voltage at the spark generator.
    i attach the board symbol
    Close-up of a circuit board in a device with visible wires and electronic components. .
  • #49 21434896
    EkoGrzew
    Level 20  
    Posts: 431
    Help: 28
    Rate: 231
    The boiler will live. The two capacitors mentioned here on the test are good, but after measuring one 100 ηF above nominal value and the other 100 ηF below nominal value. Replaced. Also checked other paths, soldered capacitor, cleaned, .
    After fitting everything, the boiler returned to operation.
  • #50 21781800
    pawelvrh
    Level 11  
    Posts: 23
    Rate: 15
    Vaillant VC PL 186/3-5, on board no. 0020035421, I replaced one capacitor C5/C30, 470 µF/35 V (it had 320 µF).
    The other same C9/C29, 470 µF/35 V, had 462 µF, remained original.
    Error F27 is gone, the oven is working again.
    Thanks to everyone who helped in the thread.
  • #51 21835737
    aismenik1
    Level 1  
    Posts: 1
    I also had an F27 error. On advice I replaced these two 470 µF. Everything works fine. Thanks.
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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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