PC maker Mesh has been forced to change its advertising after the ASA ruled its warranty was being misrepresented in adverts.

Some people who bought from Mesh expected that, when their PCs broke down, the company would offer on-site service as a matter of course if the telephone helpline could not fix the problems. But Mesh does not offer on-site service for all problems.

Mesh was found by the ASA to be in breach of two advertising codes – truthfulness and substantiation – because although the PC firm stated in its adverts phrases such as "in the unlikely event that the problem can't be solved by phone we will despatch an engineer to you", it did not do an on-site visit on every occasion.

ASA officials agreed with those who complained, that "the claim … would be understood by readers to mean that the advertiser always carried out on-site repairs if a problem could not be solved on the telephone".

Mesh has now changed its advertising by tacking on the phrase "(subject to terms and conditions, mainland UK only) or Mesh will pay carriage collection for full diagnostic checking".

"If I was a customer I would have thought that," said Geraldine Dawson, services director for Mesh. "The only time we won't send an engineer out is if it's a software problem or if it can't be diagnosed." For example, motherboard problems are notoriously hard to diagnose by phone and cannot be fixed easily on site, so Mesh insists these units are returned.

Dawson said Mesh's on-site warranty still stands because the firm will change its advertising and its product documentation, including its warranty certificate which currently clearly states that Mesh will either deal with problems on the phone or with an on-site visit, to clarify its warranty claims.