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
- Error code: F.27 = false flame signal detected [Vaillant Service Manual].
- Likely cause distribution: 95 % PCB, 5 % flame electrode [Elektroda, VPS, post #18476980]
- Typical failing capacitors: 220–470 µF, 35 V electrolytics, ×2 on lower-left PCB corner [Elektroda, Adewag, post #20557245]
- Parts cost: PLN 2–10 DIY vs. PLN 1000+ for new PCB [Elektroda, pepe__75, post #18481506]
- Board reference: 0020035421 / TM-AC1 V132_C1 [Elektroda, Piter.Krr, post #18326756]
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?
- Isolate power and remove the front panel.
- Unclip the PCB, desolder C5 and C9, fit new 35 V capacitors observing polarity.
- 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]
How much money can DIY repair save?
One user spent PLN 7.80 on capacitors instead of PLN 1022 for a new PCB, a 99 % saving [Elektroda, pepe__75, post #18481506]
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]
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 
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