A common yet often wrongly diagnosed problem with a gas oven is when you turn it on and find 3hrs later your food is raw.investigations shows the flame in the oven is there but very small.The thermostats not working ,must be faulty ? right !!! Wrong
what is actully faulty will be FFD (flame failure device)or as it is sometimes called FSD(flame supervision device)
This part is fitted to nearly all domestic gas cookers madeand sold in Britain .its function is regulate the saftey of the oven flame .
when you first light your oven it will light on a low flame called a bypass rate.When the FSD recongises the heat from the flame it allows the main burner to open fully and be regulated by the oven thermostat.If the FSD doesnt recognise the flame or is faulty then the oven will remain lit on a very small flame..
In most cases the Fsd will need changing ,NOTE these are not an item to strip and repair older ones very often contain mercury so can be dangerous to health
2 things to check before buying a new one there is on the end of an fsd the copper capilary wire &sensor the tip of which should be mounted on a clip near the burner so it senses heat ..if its not in the flame then it wont operate and will give the impression of a faulty part.The other is where there is a programmer fitted the FSd will have a solenoid attached to it and will be electricallly operated again the solenoid is part of the fsd so you cant split the two.You should however check the timer is working or that the setting havent been altered on the timer .test the live supply to the fsd .to see wether there is voltage present with the timer set on if there is change the fsd if not check the timer ,incoming fuse etc for power