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Thanks Zacs, but do the engineers always replace the filter with the service?
We paid nearly a thousand euros for a service when it was switched off last May, it then stood idle until late October when I switched it on, & immediately kept getting a warning of a fault - thought it worked OK, I just had to keep pressing the 'reset' button
This was traced to a dirty filter, which obviously hadn't been change last May.
The engineer said it was because of muck in the oil tank, but I think he was just trying to get out of not having replaced the filter when it was serviced. He went further & said the whole tank which contains 2,000 litres should be drained, cleaned & the oil put back - a massive & expensive undertaking.
I think that there will always be a certain amount of muck in a tank (as in a car) but that's what the filter is there for surely?