Furnace will not start at all; no lights or display, completely dead.

Safety: Before touching any wiring, turn off the breaker supplying power to the furnace.

• Check breaker: Locate the furnace breaker in your electrical panel. If tripped, reset it once. If it trips again, do not continue resetting it—call a professional.
• Main switch: Ensure the furnace's external service switch (if present) is in the ON position.
• Internal fuse: Remove the front cover and locate the small fuse on the Bosch control board. With power off, remove it and test it with a multimeter. Replace it only with the same type and rating.
• Terminal block: Verify that the line and neutral wires are firmly secured at the furnace's power terminal block.

If the fuse blows again or power is still absent, a shorted component or a board fault requires professional diagnosis.