What is the pyrometer used for?

[tree98]

If your engine is demandated, the ecm can no longer keep exhaust gas temperatures in check so you need a way to keep an eye on it. If Temps start getting to high you back off the throttle/downshift.

They are also very handy for troubleshooting things like exhaust restrictions and catching leaky doser injectors before they melt down your doc/dpf and cook your turbo.