ECU, check engine light (bulb/led), and all wiring in between. Note that if this is not under warranty, they’re likely to charge you a lot of money for it.
When (not if, but when) they resist checking it under warranty, explain to them that you have made them aware of a potential problem in which the check engine light does not come on when it is supposed to, and will therefore be back to see them at any time in the future that massive engine damage results from your not knowing of a problem due to the light that they should have fixed not telling you.