Vaillant boiler F67 fault code problem is board

[49mush]

Hi i have vaillant turbotech boiler.after power on display F67 code and not work anything.fan,ignite,pump etc.but when i replace different board everything works well.i am electronic technician an auto factory.i measured all voltages ok.no anything bad components.anyone know why this fault shown ? Fault code mean is This is the error code that tells about the existence of ionization voltage and therefore affecting the voltages and causing the electronic card to show a malfunction.

How works ion voltage signal on the board ?

[Swake]

This error means the ionization signal is outside of the limits of 0V to 5V.

Some ideas:
- Try measuring the signal from the sensor and see if it is effectively between these limits. Might be that the new board is more tolerant.
- There must be some sort of analog to digital convertor on the board. This ADC might be bad. Follow the PCB tracks from the sensor connector to whatever circuit is doing the conversion and try finding the fault.
- With the old board installed you can also try simulating the signal from the sensor.

[BrokenYugo]

With any fault pointing to that area the first thing to confirm is that the probe is clean. Very common problem, they develop a buildup over time. It needs to be clean so it can conduct electricity through the flame when present, that's how it works.

[floobydust]

But OP says a board swap fixes it, so that likely rules out the electrode.

The flame-detect ionization voltage is medium voltage AC. Anywhere from 24-100VAC can be used.
Water on the board can cause this to be low, or the  DC-DC converter is not running propery etc.
Can't say much without clear pictures of both sides of the board.

[49mush]

Thanks.i measure again with under microscope.

[49mush]

Which section flame sense and how works ?

[Swake]

You will have to follow the wires from the sensor to its connector and trace the pcb to see where it is terminated.
There is no schematic publicly available apparently

[fourtytwo42]

Check the flame sensor wiring for insulation cracks! I had a boiler that simply had intermittent ignition failure, gas engineers could not find it.
I discovered the sensor wire in the wiring loom had some sort of hard insulation on it that had cracked in several places down it's length (over 1M) and replacing it with a genuine spare completely solved the problem. I had also cleaned the electrode but that did not fix it.

[49mush]

Hi.cpu is fault.its cannot fix.thanks.

[JonH88]

My Vaillant boiler has a similar looking pcb.  I found that the two capacitors (looks like C.29 and C.30) associated with the 24V rail were defective (high ESR).  I changed these and the two smaller electrolytic capacitors.  I left the larger capacitor on the input side of the switchmode power supply alone as this measured OK.  I can't remember what fault code I had at the time but if you fault is in any was associated with the main 24V rail then this could be the problem.  Changing the capacitors cleared the problem.

[floobydust]

That makes sense - I test and replace electrolytic capacitors on boiler control boards because they are in a hot environment and some are powered up 24/7.
Next common cause of failure would be worn out relays, or I have seen water damage/corrosion on a board once before that got splashed.

OP saying the flame sense error is caused by a bad MCU seems off - the board uses dual MCU's for functional safety. It has CDI ignition. The flame sense circuit is another section and I did not reverse-engineer it. There are many different, patented ways for flame sense and the safety standard requires a self-test of all its components.

[tom66]

A bad ADC on one of the two MCUs could cause a disagree fault between the two MCUs which would lock the boiler out.  I'd assume flame detection is an essential process that is monitored along with heat exchanger temperature, gas valve position and perhaps gas pressure.

[darkspr1te]

Years ago I help a friend in uk fix his gas boiler, all the engineers had done the usual board and probe swaps and the actual issue turned out to be the probe was in the wrong place, this unit used a spark gap to ignite the pilot flame and then measured the two wires of the probe and if the tip was not in the flame then it would not heat and shut off the gas, if it was too far forward it would not ignite and too far back it would ignite but the tip would not be in the flame. And the reason it had failed initially was that when they moved in by law a british gas inspector must sign off that the system is safe and when he pulled the ignitor for inspection it was not seated back right.
Of course the usual finger pointing happened and my mate was blamed for the issue at first but then after finding the ignitor was fitted wrong he was able to claim back the fee's.
so if your system has a ignitor style pilot flame control probe that could be the issue.


darkspr1te

[cncjerry]

I would replace the electrolytics.  Went around this with my furnace.  Then two years later found a cracked PCB trace, fixed that and all is well. You might want to spend some time with a continuity tester and just go from pad to pad to see if you have a crack before the electrolytics are swapped out.  Mine were obviously leaking.  some of these boards are exposed to temperature extremes.  Why they mounted the furnace board in the air box on mine is beyond me.

Jerry