Furnace displays E04 and will not start; may occur immediately after power-on.

Safety: Turn off power and allow the furnace to cool completely before touching internal parts.

Steps:

• Reset the lockout: Turn the main power switch OFF for 1 minute, then ON. If E04 reappears quickly, continue with checks.
• Check air flow: Ensure all supply and return grilles are open and not blocked by furniture or dust buildup. Replace or clean any external filters used with the Cointra furnace.
• Inspect circulation fan: Remove the front panel and visually inspect the main circulation fan (blower). Check for dust buildup, seized bearings, or foreign objects. Spin the fan wheel by hand (power off) to confirm it moves freely.
• Clean heat exchanger surfaces: If accessible, gently vacuum dust from around the heat exchanger fins and internal air passages. Do not bend fins.
• Check high limit thermostat: Locate the high limit thermostat on or near the heat exchanger. Ensure wiring is secure. Some models have a manual reset button; if present, press it once to reset.

Professional help: If the furnace continues to overheat, a technician should check gas input rate, verify fan speed settings, inspect the heat exchanger for internal blockage, and replace the high limit thermostat if it is weak or defective.