Depending on the specific furnace serial number

If the direct spark ignition module is powered but there is no visible arc at the 11J38R06891-003 probe

Enhanced System Safety: Prevents terminal heat damage by interrupting the gas valve circuit during restricted airflow conditions

What causes a thermocouple signal to drop below the required 20 mV

if voltage is present but the SPST contacts do not close or the timing interval is non-existent

Carrier HH22HB075 T-Stat 30067439-890 Depending on the specific furnaceThe Carrier HH22HB075 is a professional grade outdoor air thermostat designed for precision ambient temperature monitoring in residential and commercial HVAC systems. This fixed point temperature switch is critical for managing heat pump balance point settings, effectively locking out the heat pump or engaging supplemental heat strips when outdoor temperatures drop below the optimal efficiency threshold. Engineered specifically for Carrier heat pump