What are the different causes of heat loss in an oven?
The list might vary a bit based on whether you own a gas or electric oven. However, there are also many common issues between the two which should be checked for if your oven doesn't seem to hold the right temperature.
Firstly, check your oven door seal (gasket) to see whether there are any gaps or breaks. Hot air can escape this way and the oven might not be adequately compensating for the loss. The same can occur if your oven door hinges are not in place. Some ovens are worse for this than others, but in some models the hinges can adjust their positioning when opening your oven door.
Some ovens will fluctuate temperatures more than others. A rule of thumb is to allow for up to a 50 degree difference in total; if you set your oven for 450, the temperature could bounce between 425 and 475 with the intent of trending toward 450 degrees Fahrenheit. As parts wear out the ability to maintain this range can suffer, but sometimes calibrating your oven will do enough to remedy the heat regulation issue.
Of course, many damaged and defective oven parts can cause heat control issues. Some examples include the temperature sensor, control board, heating elements and gas safety valve. Unfortunately, a DIY oven repair will be almost impossible for the average person if the problem lies within one of these internal components.
You should check the minor causes first but if that doesn't solve the problem you will need to diagnose internally. To do so on your own, a multimeter is required - keep in mind, the live voltage testing that's sometimes required to troubleshoot heat control problems should be left to a professional.
Are you making horrible cakes when you use your convection oven? The reason for this is because your oven is using a circulation fan that delivers the hot air throughout the inside. The result of this is a reduction in the amount of rising that occurs; the heat distribution does not work in your favor. For baking cakes, it's essential to utilize the conventional baking mode instead. Additionally, the convection mode can achieve the same cooking results as a conventional oven, but at a lower temperatue - lower the cooking temp by roughly 10% when using the convection mode.