My covection oven does not heat up once food is put in

On the gas valve there is an adj. for the pilot size, maybe too small , would try increasing the pilot and see what happens.also the thermocouple might be on the way out ,you might want to replace that and try oper. go with thermocouple first, then try pilot size dont forget to vote tks. monomoy

Gn 902's have two pilots , one on each side, if I am not mistaken. Each pilot has a flame sense with a capilary back to the switch itself.( front center with the burners) There is a switch in the convection motor that will have to close, and you have door switch interlocks.
Golden rule of troubleshooting is " What's changed?"
Is ok to pre heat, then fails. Oven gets hot and may expand, dropping a door switch closing. ( minor adjustment) Check with a meter first. And the doors are opened and closed ( door switch again) The thermostat will drop due to doors open and cold product loaded. Monitor the Tstat output with a meter. Check if you have a load switch that it is set for continuous, to troubleshoot and check it's output.
Disconnect power to remove control panel. Some bare contacts will short to chassis while removing panel..