During a no-heat call, if the thermostat terminals read 0V and the gas valve coil terminals read 0V, what component could be open?

In a no-heat scenario, when both the thermostat terminals and the gas valve coil terminals read 0V, it indicates that there is no voltage present at both the control and the heating components.

The high limit switch, which is a safety device, can prevent the furnace from operating if it senses that the temperature is rising too high, thus interrupting the circuit. If the high limit switch is open, it would effectively break the power path and result in the gas valve coil not receiving the voltage it needs to open. This situation aligns with the readings of 0V at the gas valve coil terminals and the thermostat terminals, as the open high limit would stop the system from functioning, causing the voltage to drop to 0V.

Hence, when looking for the component that might be open in this scenario, the high limit switch is the most plausible answer as it can interrupt the operation of the heating system under certain conditions, leading to a no-heat situation.