How can I fix my Dishwasher. Smeg DWi 710E?

I have an error message, which according to the manual, means it cannot heat the water. I had an engineer out once, and after cleaning the filter, he reached under the machine and said that the machine had a trip switch to prevent the machine over-heating, if there was insufficient water. I've cleaned the machine myself but cannot find any trip switch. Is there one, or is there another method I can use to fix it myself? Thanks.