Oven is taking to long to heat

Isn't the upper element the grilled? I don't think it is controlled by the oven thermostat.

Not sure how your oven works, but older ones had a "pre-heat" setting that you would use to get the oven to temp. This ran both the upper (broiler/grill) and lower elements at the same time to raise the heat quickly to temp. Once the oven is pre-heated, you would switch the selector to "bake" and that would only allow the lower element to work after that. If you failed to remove it from "pre-heat", it would use both and burn the food. Just saying.

I would check the oven sensor it may be defective

Oven comes on and off intermittently or heats very little:

If the timer feature is activating and you have not touched the timer button at all, this would have to be a failed Electronic Oven Control. The timer button is either shorting at times or closing on its own from heat or moisture. The Electronic Oven Control would need to be replaced to repair the problem.

Or Why does it take the oven so long to bake?
When the food is taking way too long to bake, it's probably a weak bake ignitor. Replacing the ignitor usually fixes this problem, but you probably want to verify that the ignitor is the problem before replacing it.

Sometimes the oven thermostat or oven sensor can be calibrated wrong, or it may be faulty. If your particular range has an oven that uses an electronic thermostat, and the oven temperature is off by tens of degrees, you probably have to replace it.
On most units that have a mechanical thermostat, you can actually remove the thermostat knob, and adjust the knob to more accurately represent the actual setting of the thermostat. On many models, there's a screw on the back of the knob with a small calibration plate or ring. You can loosen this screw and adjust the calibration plate. Remember to tighten the screw again. If yours isn't adjustable, and the temperature is off by a large amount, you should just replace the thermostat.

Or Oven safety valve needs to be checked with multi meter ohms / voltage

ALSO Test the Burner Heating Element The stove's burner heating element is a coil of metal sheathed in an insulator. Electrical current travels through the element. Resistance to the passing of electrical current causes the element to heat up. A precise temperature cannot be set for a burner, instead it is turned on and off repeatedly by the control to the achieve an average temperature. When it is set to a low temperature, the element is cycled on and off more frequently. For high temperatures, the heating element is energized longer with fewer on and off cycles. Some burners have two elements, with the second only being used only for high heat settings.
Before testing the heating element, unplug the appliance or shut off the power at thefuseboxorbreaker panelto avoid an electrical shock hazard.
When a burner does not heat at all, or only heats up to a lower than expected temperature, the problem is likely to be with the heating element, the temperature control switch, or the wiring. If it only heats at the highest temperature, the problem is with the control or an electrical short, not the burner. If the burner works only intermittently, the problem is likely in the wiring or connectors. To test the heating element, try the following steps.
First, disconnect the heating element from the stovetop. In most cases, this is done by lifting up the burner on the side opposite of the terminals (the part of the burner that disappears under the stovetop). Remove the decorative ring.
Inspect the style of connection. If the burner element has visible blades that fit into the receptacle block, pinch the block with one hand, and pull the heating element free with your other hand. If the terminal block clamps over the element, the housing must be removed and the burner wires disconnected. Unsnap the metal piece or remove the screw that secures the receptacle block and then disconnect the element.
Inspect the heating element. If you find bubbles, warping, or damage to the insulation sheath, the burner must be replaced. If the terminals are dirty or corroded, this can cause poor temperature control, intermittent problems or complete failure to heat. Clean the terminals with steel wool or very fine sand paper to restore good conductivity.
Test theresistanceof the heating element using amulti meter. Set the multi meter to the ohms setting X1 and touch one probe to each of the terminals. A normal reading is typically somewhere between 20 and 120 ohms. The exact reading differs by manufacturer and mode. If the meter reads infinite resistance or the other extreme of the scale, zero resistance, then the element is damaged and should be replaced. If the measured resistance differs significantly from the expected range, the element is probably bad, but if possible, determine from the manufacturer what the actual resistance should be.
To test for a grounded or shorted element, touch one probe to the surface of the burner and the other probe to each terminal in turn. If you get continuity at any time, the heating element is defective and should be replaced.

I have a Kitchen Aid convection wall oven. It takes 6 - 7 minutes to preheat to 325 degrees. You probably have a bad element. There are 2 elements.

This tutorial will show you how to check your Smeg element and how to replace your Smeg cooker element.
Here are the 3 main faults to look for in your oven or cooker that is not heating

1. Your oven will not work until the clock/timer has been re-set after replacing an oven element or tripping the electrical supply. Consult your Smeg manual.
2. If your Oven is not getting hot or heating up it could be a faulty element, if the fan in the oven is running and the thermostat light is on, this will lead us to believe that the element has gone faulty. With grills the thermostat light should come on there should be heat, if not then this could be the grill element or the thermostat.
3. When you turn the oven on, the household electrical supply trips on the Rcd (this is the earth trip in the fuse board sometimes called the breaker tripping) your fan oven element has gone to earth and needs replacing.

If this helped you why not get me a beer? Paul is from the Naturally he would appreciate beer How to Repair Home...

The electronic timer must be in manual mode for the oven to work. Turn the oven on. Play around with the clock/timer buttons using as many combinations as you can. When the oven indicator light comes on the clock/timer is in manual mode. This could take 5 minutes or more. Be patient.

To pre-heat an oven is to simply turn it on to the desired temperature and wait 5 to 10 minutes. If you have a timer then set the timer for the desired time you want to pre-heat then begin baking as usual.

There is no magic to pre-heating an oven and everyone that bakes has a different opinion.

Try unplugging the oven and let sit for 10 min. This will reset the curcuit board and reboot the electronics. You may have to pull out the stove o get to the cord.

check incoming electric first, pull it out and gain access to main wire terminal, With multimeter set to at least 250v AC , check your power across the wiring as such. black to white (110v), red to white(110v) and then black to red 230v. if that ok ok turn, then start ckin elements for continuty also there might be a thermostat( usually an 1 in. round component with 2 wires located right inbetween the elements that has gone bad, will show no continuity. ck that before electronic control the fact that its clickin sounds like the relays are workin properly

It looks like an S551X-5, I think the timer is a safety mechanism, maybe just set it to full when in use and turn it off when you finish.
Go to http://www.smeguk.com/KitchenStudioLine/Product/S551X-5.aspx to see features, etc.