Furnace shows E08; inducer and blower may not run; no response to thermostat

Safety first: Turn off power at the breaker before working on wiring or the control board.

• Check board fuse: Remove the blower door and locate the low-voltage automotive-style fuse on the Napoleon control board. If blown, replace with the same amperage rating. If it blows again immediately, there is a short.
• Inspect thermostat cable: Follow the thermostat cable from the furnace to where it enters the wall. Look for staples through the cable or damaged insulation. At the control board, disconnect thermostat wires (R, W, Y, G, C) and inspect for bare copper touching.
• Isolate the short: With thermostat wires disconnected, replace the fuse and restore power. If E08 clears, the short is in the thermostat or field wiring. If E08 remains or fuse blows, suspect transformer or internal board short.
• Check transformer output: With power on and extreme caution, measure AC voltage across the 24V secondary terminals of the transformer. It should read around 24 VAC. If zero, the transformer is likely failed.
• When to replace: Replace damaged thermostat cable, the 24V transformer, or the control board as indicated by testing. If you are not comfortable with electrical measurements, contact a technician.