E4 code, furnace lights but shuts off quickly, repeated retries then lockout

Safety first: Turn off power and gas before removing burner or flame sensor components.

• Step 1 – Clean flame sensor: Remove the flame sensor from the burner assembly. Lightly polish the rod with fine steel wool or emery cloth to remove oxidation. Do not sand aggressively. Reinstall and tighten screw.
• Step 2 – Check ground connections: Verify the green ground wire from the power supply is firmly attached to the furnace chassis. Ensure burner rack mounting screws are tight and free of rust, providing a solid ground path.
• Step 3 – Inspect burners: Remove the burner assembly if accessible. Look for rust, debris, or spider webs in the burner ports and crossover channels. Clean with a soft brush and compressed air. Do not enlarge burner ports.
• Step 4 – Confirm gas pressure (pro only): If flames appear weak or unstable, a technician should measure inlet and manifold gas pressure at the Ducane gas valve and adjust if necessary.
• Step 5 – Reset lockout: Restore power and gas, then cycle the thermostat off and on to clear lockout. Observe operation. If E4 persists, the flame sensor or control board may need replacement.