What This Error Means

Error code 42 means the inducer did not start within the required time, operated outside its valid RPM range, or lost its RPM feedback signal during operation.

Symptoms You May Notice

Furnace will not proceed to ignition; inducer may not start, may run erratically, or may stop; pressure-switch-related symptoms; repeated startup attempts; code 42 displayed.

Common Causes

Failed inducer motor or electronics; loose or damaged inducer harness; obstructed inducer wheel; water in the inducer or collector area; blocked vent or intake; unstable line voltage; failed furnace control; lost RPM feedback signal.

Safe Troubleshooting Steps

1. Turn off furnace power. 2. Check accessible vent terminations for visible obstruction. 3. Look for water leakage around the furnace and condensate system. 4. Listen during one startup attempt for humming, scraping, surging, or no inducer operation. 5. Do not insert tools into the inducer or bypass pressure safeties. 6. A technician should test inducer power, RPM feedback, wiring, venting, condensate drainage, wheel condition, and control-board outputs.

Parts Commonly Involved

Variable-speed inducer motor; inducer electronics or RPM feedback circuit; wiring harness; vent and intake piping; condensate system; integrated furnace control.

When to Call a Technician

Call a technician when code 42 appears. Inducer RPM, electrical communication, venting, and condensate conditions require model-specific testing.

Safety Notice

The inducer is part of the combustion-venting safety system. Do not bypass related controls or operate the furnace if you suspect blocked venting or combustion-gas leakage.

Manufacturer Documentation