An oven rarely dies suddenly. It drifts, and the drift goes unnoticed until everything comes out wrong.
Check the temperature twice a year
Put an oven thermometer on the middle shelf, set 350 F and leave it half an hour. Anything more than about 25 F out means the sensor has drifted. Left alone it keeps drifting, and you keep adjusting recipes to compensate without realising why.
Go easy on self-clean
Self-clean is the hardest cycle an oven ever runs, and door locks, temperature sensors and control boards fail during or shortly after it more than at any other time. Use it sparingly, and never the day before you need the oven.
Look after the door seal
The gasket around the door is a consumable. When it hardens the oven leaks heat, the element runs longer to compensate, and everything else on this list gets worse. It is a cheap part and a quick fit.
Do not line the floor with foil
It reflects heat back at the element and blocks the airflow the oven was designed around. On some models it also traps heat against the sensor, which then reads high and underheats everything.
Listen to the preheat
If preheating takes noticeably longer than it used to, a gas igniter is weakening or an electric element is partly failed. Both still work, just not well enough, and both are ordinary same-visit repairs before they stop entirely.
