If the work we did is not right, we come back and make it right, and we do not charge you a second time to correct our own error. That is the whole guarantee. It covers the workmanship and the way the job was carried out: the installation, the connections, the commissioning, the cleanup and the explanation you were given. It applies the same way to a maintenance visit, an emergency call at 2 AM and a full system installation.
What it does not do is convert a repair we advised against into a free replacement, and it does not cover a separate failure elsewhere in the house that we identified and told you about at the time. No honest company can promise that, and a guarantee that claims to is a guarantee nobody intends to honor. The value is in the part we can actually control, which is our own work.
The practical part is telling us early. A problem raised the week of the installation is a callback and it stays simple. The same problem raised two years later, after another company has been into the system, is much harder to sort out for everyone involved. Call the office, describe what the system is doing, and ask for it to be logged.