Problems:
Problem 1: You have not told us the make and model of grill or even if it is gas or electric.
Problem 2: You have not done a Google search for - say - "cooker grill only comes on low". You'd have found results like these:
https://www.sussexcookers.com/index.php?route=advice/supportflamefailuredevice
http://www.ukwhitegoods.co.uk/help/fix-it-yourself/ovens-hobs-a-cookers/2821-basic-fault-finding-ovens-and-cookers
http://homeguides.sfgate.com/troubleshoot-gas-grill-low-flame-41895.html
acp, July 2017
