Error code 04 and furnace cycles the inducer and igniter but gas valve never opens

Safety: Turn off power before removing blower door or accessing wiring.

Error 04 indicates the high limit switch is open, preventing burner operation.

• Check air filter:
  • Locate the filter in the return air cabinet or filter rack.
  • Replace a dirty or collapsed filter with the correct size and MERV rating recommended by American Standard.
• Inspect supply and return airflow:
  • Open all supply registers and return grilles.
  • Remove any furniture or rugs blocking airflow.
• Reset or test the high limit switch:
  • Locate the high limit on the heat exchanger panel (usually a disc with two wires).
  • Some models have a manual reset button; press gently if present.
  • If the switch repeatedly opens, use a meter to test continuity when cool. Replace with an OEM American Standard limit switch if it fails.
• Verify blower operation:
  • During a call for heat, ensure the indoor blower starts and runs at the correct speed.
  • If the blower is slow, noisy, or not starting, the blower motor or capacitor may be failing and should be serviced.