I cooked something for 40min as it said and at the end it was raw, I had to pur it on further 40 min this is a conventional oven

Hello Linda;

My name is Peter. I am a retired field service appliance technician.

The normal operation of a oven, with the exception of the new over priced accurate ovens: The oven will raise 20-40 degrees above the set temperature and then turn off. When the oven temperature drops 20-40 degrees below the set temperature it turns back on and the cycle repeats.

There are 3 components that affect your oven operation:
1. Burner element - Although your burner element is providing heat - inspect the two element connections in the back of your oven for corrosion. Here you may not be getting enough power through the burner element.
OK. No corrosion.
2.) Thermocouple - Inside your oven is the thermocouple. It is either a rod coming straight out in the back upper right corner or fixed against the back wall. It is a little larger than a coat hanger wire. The thermocouple communicates directly with the burner control. The burner control will raise or lower the temperature based on the feed back from this sensor.
3.) Burner control will increase or lower the electrical power or gas supply based on the reading from the oven thermocouple.

A thermocouple purchased over the internet, depending on what you have is between $35-65.00.

The oven control board - I recently replaced a oven control board for a friend. His oven temperature was off 200 degrees. Sears wanted $256.00. I got a remanufactured oven control board over the internet for $30.00.

If you are handy, both of these components are easily replaced.
If I were still working for Sears Home Services and came to your home, the bill would be the parts x 2 plus and about $200.00+ labor.

You did not provide your model number, which is on a metal label on the face of your oven when you open the door. So, I can not give you the part numbers that you need.

It could be your oven isn't as hot as you set it. Get a thermometer and check it. It could be the "something" you cooked was larger than the recipe was expecting... It's cooking, you have to adjust. It could even be your altitude above sea level. Higher, water boils at a lower temp, things take longer to cook.