Vaillant Thermocompact breakdown, error / fault code F.28

I got called to a Vaillant Thermocompact system boiler this week. Symptoms were as follows:

– boiler losing pressure over 24 hours period, client had to top it up daily
– with correct system pressure at between 1 – 1.5 bar boiler would start and run for 5-10 seconds (burner starting) and then boiler would go to lock out and display an error code F.28

Picture below is from a similar fault on the same boiler model. (I forgot to take pictures this time)
vaillant thermocompact plus

According to the manual error code F.28 could mean following:

– no gas
– isufficient gas
– incorrect gas valve
– electrode defective
– ignition lead defect
– ignition defect

repairs to the boiler
– clean and align monitoring electrode
– replace PRV, replace schrader valve on expansion vessel, replace lime scaled auto air vent
– top up expansion vessel to correct pressure

Please note that in most cases it is necessary to remove this boiler off the wall to replace faulty expansion vessel making this repair quite expensive. This boiler is very similar to Vaillant Turbo Max Plus and all of the above would work exactly the same on Turbo Max Plus model.