Hi there Iv'e got a 55 plate focus 1600 tdci, the engine system fault light came on 6 months ago turned out to be the fuel additive resevoir tank was low or empty. mechanic sorted problem same thing has happened again. my question is can any damage occurr to engine etc. if the fuel additive tank is not topped up when required. also how often should the tank need to be topped up

thankyou