The preheat cycle on an oven is a simple setting that allows you to ensure that the oven has reached the right temperature. The oven will turn on both burners and will gradually heat up until it reaches the set temperature. Ovens that have a preheat setting generally have an indicator light or a signal that shows the oven has reached the set temperature. Some ovens get stuck in the preheat setting and never reach the right temperature or they take too long.
Ovens that aren't installed properly may not ever be able to reach very high temperatures such as those needed to bake pizza or make thick crusts on bread. This occurs when the installer uses the wrong voltage. Gas ovens should generally be set around 120 volts, and electric ovens should be able to use 240 volts. If the oven is below 105 volts it will not be able to preheat properly.
Broken Heating Elements
Many ovens have at least two heating elements that will both turn on to gradually heat up. If one is broken, the oven may get stuck in the preheat setting and be unable to heat up enough to reach high temperature or maintain medium amounts of heat.
Weak Heating Elements
Electric heating elements will wear out over time and generally will be noticeably weak before they break. Often this occurs in both heating elements at the same time and could prevent the oven from heating up efficiently.
Broken Thermostat Bulb
Each oven is equipped with a thermostat bulb that will test the inside temperature. If the bulb is broken it won't be able to register the correct temperature and will not send the correct temperature to the control panel that will heat up the elements. Test the thermostat bulb with a multimeter to determine whether it is broken.