Bottom oven does not heat up fan works

either a bad connection on the element terminals or faulty element

this should show you how to change it

You-Tube video "pEhRqdvpiJM" placeholder (do not delete)

The main fault when the oven is not heating is that the element has blown here is a video on how to change your fan oven element and the motor.
For all your fan oven element click here

If the fan is working o/k, then you need to replace the oven element, located behind cover at rear of oven compartment, very straightforward repair to do.
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You need to replace the oven fan element, as there is probably a seperate element in use when used on convection heat.
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Thanks.

The heating element is the likely culprit as they burn out and go open
circuit..(ie ...disconnected and will not heat up )
If you can use a multi-meter , then the element can be tested using the resistance range (with the oven SWITCHED OFF )

Yes it will need a new element, if the fan runs and temperature light comes on then the element is gone.

No resistance in a bake element is a bad element. Replace it.
Most fan come on with a rise in heat. No heat/no fan.

the ring element may have blown. if it works ok with the top and bottom elements, (the convection oven) then it is likely the element surrounding the oven fan.

there is more to this than meets the eye if youre oven takes a long time on fan setting it may be thermostat but more likely to be fan not turning at all or quickly enough the stat capillary is above oven element and if fan not blowing air away from capillary oven stat will shut oven temp down too soon if using conventional heat eg no fan make sure there are no full size trays directly on bottom of oven this will block heat from bottom element effectively giving only half the heat output.

hope this helps