Control board LED flashes code 23; furnace may start but shuts down unexpectedly or will not start.

Safety: Turn off power before working inside the furnace. Do not bypass safety switches for normal operation.

• Step 1 – Inspect tubing: Locate the pressure switch and its rubber tubing to the inducer housing or collector box. Remove the tubing and check for water, debris, or cracks. Clear any blockage and replace brittle tubing.
• Step 2 – Check venting: For high-efficiency Tempstar furnaces, inspect PVC intake and exhaust pipes outside for obstructions (ice, leaves, nests). For standard models, inspect the metal flue for blockages.
• Step 3 – Test pressure switch operation: With power off, disconnect the wires and test continuity across the pressure switch. At rest, it should be open. During inducer operation (power on, call for heat), it should close. If it remains stuck closed or open, replace the pressure switch with the correct Tempstar-rated part.
• Step 4 – Inspect inducer motor: Ensure the inducer motor starts promptly and runs smoothly. Listen for grinding or slow startup, which can prevent proper draft and pressure switch operation.
• Step 5 – Verify condensate drainage (condensing models): Check the Tempstar condensate trap and drain lines for clogs that can flood the collector box and affect pressure readings. Clean and flush as needed.