Vaillant ecoMax Pro not firing, no error codes

Hi all, I'm hoping someone here can point me (and my heating engineer) in the right direction with this.
I have a Vaillant ecoMax Pro 28e boiler installed and for years she has done us proud. Recently however, it's developed an odd fault. Every 3 or 4 days it just refuses to fire, but doesn't give an error code. Initially I managed to get it going again by turning it off and on again, but now that had failed me too.

When the issue occurs, and I have restarted the boiler, it goes through the standard start up stuff. The heat/water symbol clicks in and I can hear the system pump running. Other than that though, I get nothing. After about 30 seconds, the pump cuts out again. When it does eventually decide to playball - usually after having been turned off for several hours, and the immersion heat having been turned on, the boiler sparks right away and you can hear the fan running. As I mentioned, the most frustrating part is that there are no codes thrown. We have even checked the installer menu and there are no faults stored there either.

Our regular heating engineer has been out and suggested we change the mainboard, new board ordered from Vaillant and fitted, but it didn't make a difference. The motorised valves have all been checked and are working.

I'm wondering if the transformer for the fan is giving up the ghost, but understand from reading other threads that the newer boards have the fan transformer onboard - so the new board would have fixed it?

Obviously it being gas I am not about to pull my boiler in bits, but any previous experience that I can provide my engineer with would be greatly appreciated.

I should mention the boiler behaves the same with the central.heating and the hot water. Tries to do something when it first starts up, but then falls silent and ignores any further calls for heat.
Thanks
Dan

If it is only 4 years old it will still be under warranty get them out to fix it

Sadly it's older than that.

In a further update, we've had our engineer out again and he replaced the option board, but no change. He spoke to Vaillant who had him checking various parts of the boiler. The supply to the option board (green connector) appears to cut out when the pump goes off after the boiler initially starts up.
He has now advised us to replace the boiler - which feels like a "I have no idea how to fix this" moment.
Frustratingly, the boiler appears in good condition and I am loath to get rid - especially when it will leave me 2k lighter.
What are the chances that the main PCB that was sent out by Vaillant was faulty? This is feeling more like something iffy is going on with the electronics...

Its extremely easy to get pcb edge connectors wrong on these boards, some go one way up and others downwards. get you heating guy to treble check he has all the connectors on correctly.

Double check the connector at the top middle of the image.

https://www.diynot.com/diy/attachments/ecomax-pro-28e-pcb-jpg.107023/

If that's ok then take a look at the connection box underneath the boiler.
Terminals 2 & 3 are the Switched Live (from the motorised valves) and the frost thermostat (if fitted).

If fitted, disconnect the frost stat wire and move the Switched Live wire across onto that terminal ie. we want to see if the controls will fire up the boiler in frost mode.

This will prove whether there is a problem with the "230 controls interface" pcb.

This small pcb isolates the 230V fed from either a zone valve or frost stat to the boilers main pcb via an optical isolation barrier.
Sometimes the resistors feeding the opto coupler fail. There is one each for the zone valves and frost stat.

Maybe Vaillant fixed price repair?

Thanks for your replies. I have had a look at the PCB and all the connectors are attached as per the photo Gasguru was kind enough to post.
I wasnt able to get to the connection box last night - i will try this evening. However, I am assuming that the 230 controls interface board is the same as the "option board"? This has just been replaced (post the issue occuring).
One other thing to add, is that when my heating engineer was trying to trace through the issue, he was monitoring voltage at various parts of the boiler and system. He observed that when the boiler starts, the symbol for heating/water demand clicks on, and the boiler appears to start the pump. At that point, the switched live feed had 230 volts. However, after about a minute of pump noise (and i think very quiet fan noise) there is a very audible click from the main PCB (as there always is whenever the boiler starts/stops), and the switched live feed goes dead. However, the symbol for heat/water demand is still on the display. He was measuring the voltage on the incoming side of the boiler - so it hadn't by that point touched any part of the boiler electronics.
We have checked the motorised valves and they appear fine - at this stage i am wondering if it could be something to do with the ignition on the boiler and something timing out if the boiler doesn't light in a given time? or maybe the controller going odd? Neither of us can understand why the switched live feed would go dead.
As always - any suggestions greatly received. I'm loath to go down the Vaillant fixed price route - as they appear to have the "beyond economical repair" get out.

Bypass the external controls to make sure its not a external (to the boiler fault).

Whilst there is a demand from either hot water or heating controls there should always be a 230V at the switched live terminal.
I don't have much confidence in your heating engineer.

So this evening I have finally managed to do some checking of the wiring as suggested by gasguru. Following the boiler having fired up for the last two days, today it decided again that it was not in the mood.
So, first I opened up the wiring box underneath the boiler and swapped the incoming switch live onto the frost stat in on the boiler. Turned it all on but nothing. It didn't even register that the feed was coming in.
Next I got the multimeter out and checked the incoming voltage to switch live - a steady 230v.
Out of interest, I opened the flue inspection point while the boiler was starting up, and could feel air coming out, which would suggest the fan is running fine.
So now I am at a loss. It's definitely something in the boiler, but as to what I have no idea. We previously tried replacing the mainboard and that didn't seem to make a difference, but could the replacement have been faulty? It's almost like the electrode isn't working or the ignitor unit has failed, but according to the manual that should throw an error.
I've put a link to the boiler starting up. You can hear the boiler click as it registers demand, and then it stop cycling after about 30-60 seconds. There is a slight clicking/squeaking noise coming from the mainboard that you can hear if you then the sound up.

http://imgur.com/gallery/EMzFiqJ

As always, any suggestions gratefully received.

Please, for the sake of putting others at risk, get a different engineer or a Vaillant fixed price repair. If it was the fan it usually throws up an error code, if anything else would sometimes throw up an error code.

Press and hold the mode button till the rad symbol is displayed and then use the plus button to raise the flow temp to 75c.

Thanks for the update mate, nice to know you got it sorted, nothing worse than being left hanging not knowing if it was fixed or not and closes the thread . oh wait you couldn't be @rsed , ah well never mind.

Alright tiger - keep your hair on. In fairness - i thought you were taking the pi$$ with the above advice.
In the end i called out Vaillant for a fixed price repair - once they have diagnosed and (hopefully) fixed it, i will update the thread, as i agree with you - nothing more frustrating than a thread without a conclusion.

Ok, the Vaillant engineer has just been and gone, and its not (for me at least) good news.
The flue collector at the back has developed a crack (which apparently they do with age) and has leaked condensate into the boiler. This has then turned to steam and cooked all the electronics in the boiler. They have said its beyond economical repair - would need new heat exchanger (as they can warp), new fan, flue, flue collector, etc.
So, new boiler time it is