A contract dispute usually turns on what was reported, when, and what the provider did next. Memory is weak evidence. A running log is cheap and puts the sequence in one place.
What to write each time
Write the symptom in the words you would use to describe it to a repairer, the item it happens on, whether it stopped the item or only affected it, and the time of day. Write what you did next, including any step you took yourself, such as a reset or a shutoff. Keep the reference of every request or call.
Who you spoke to
Note the person you reached, the company they work for, the way you reached them, and what they said the next step was. If the contact was by message, save the message. If a visit happened, note who arrived, what they looked at, and what they wrote down. If nobody wrote anything down, write that too.
Keep the log with the paper
Store the log with the contract, the receipts for any repair you paid for, and the serial or model label of each covered item. A log kept apart from the contract is easy to lose. A log that names the contract's own section for each step is easier to match against it later.
What the log does not do
A log records what happened. It does not decide whether a repair is covered, and it does not promise that a claim will be paid. Check each entry against the contract's own terms and the notice steps it sets out.