why RPM drops

[alborz]

2015 pererbilt 579
ISX 2350
500 k miles
recently it happens several times that after starting the truck the rpm gauge falls down to zero although the engine is idling.then the orange engine light comes on. if I turn off the truck and restart it again it is as it should be.
I'm wondering if there is sensor going bad or a something else.
your input greatly will be appreciated.

[Signature620]

If the engine light comes on then there will be faults stored in the ECM. Plug into it and check instead of guessing. Could be data link related.