Those of us with a Scanguage have notice the CAT temperature on a FEH is in the normal operating ranging within a very short time frame--less than 30 seconds. So this is not the reason the FEH's ICE continues to run after startup. The scanguage reveals the engine temperature CHT (cylinder head temperature) needs to exceed about 188 degree F and the radiator tempeature fWt needs to exceed 154 degree F prior to EV after a cold start prior to allowing EV/ICE shutoff.

Can someone with a Scanguage and Prius provide similar data?