Check the ignitor to see if it's ashy or white replace it if it is. Testing the amps it's pulling if you have a amp clamp multimeter it should pull 3 amps or more if it's not replace it. Also check the wires going to the ignitor and make sure you have 120v going to ignitor if you don't then your main board might need replacement. Also ohm test the temp probe it should be around 1k at room temerature and heat the oven up to 350 degrees and unplug the temp probe and ohm test it, should be around 16k ohms if it's not it may be faulty or bad. Good luck here's some helpful video's
How to Fix a Gas Oven that won't Heat - YouTube
Oven Sensor Test - YouTube