correction Dave it is controlled by ECM but is listed as, exhaust aftertreatment glow device!
its function is to reduce the fuelling measures required to re-gen and so avoiding oil dillution & the need for extended drive cycles to raise exh temp
it is literally a fuel line pre-filter with a glow plug & when required hey presto a big fire in your exhaust.
thanks for your nudge dave, sometimes a bit of self study is needed