In this context "light-off" is being used as a specific, technical term. If you are in charge of coming up with the term in the first instance (if this is the first translation of the manual into English, for example) then something better is possible but if this is a widely used and understood term then changing it will only cause confusion. It's far more important to be consistent than to have the best possible terminology, especially when an existing term is in use and is widely understood.
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