You only pay for the work that is carried out. And only if it fixes the initial problem that the Engineer was called out to fix.
What "No Fix, No Fee" cannot cover is problems where hardware has been physically damaged, and the cost of repair is prohibitive. Or where the client decides not to progress with the repairs due to cost. There will still be a charge in this case for the labour involved with the diagnosis.
A similar situation may occur where the PC is running with Pirated software or a Pirated Operating System. In this case, the Engineer will refuse to reinstall non-licenced products. He will still work on the PC, but can only perform legal installations.
There is a similar rule with all illegal acts the Engineer may be asked to carry out. So please don't ask him to supply pirate software or download an illegal movie for you.
The "No Fix, No Fee" policy cannot apply for cases of Data Recovery, Third-Party issues, Viruses or Spyware infections. Due to the nature of these issues, these can be unpredictable. The Engineer has a lot of experience in these areas and can kill problems that most Anti-Virus programs cannot deal with - but he is not a miracle worker.
The Engineer also reserves the right to not complete work when the client becomes abusive or has to work under harsh conditions.