Outdoor unit runs but indoor temperature does not drop, control board shows E4, suction line may be very cold or frosted

Safety first: Turn off power to the indoor unit before accessing the blower or coil. Coil surfaces and sheet metal edges can be sharp—wear gloves.

Steps:

• 1. Replace or clean filter: Install a new filter of the correct size. Avoid overly restrictive high-MERV filters unless the system is designed for them.
• 2. Check blower operation: With panels in place, call for cooling and verify strong airflow from multiple supply registers. Weak airflow may indicate a failing blower motor or dirty blower wheel.
• 3. Inspect evaporator coil: Remove the coil access panel. If the coil is iced over, turn system OFF and run FAN ONLY until all ice melts. Place towels or a pan to catch water.
• 4. Locate coil sensor: On many Armstrong Air systems, a thermistor clips to the evaporator coil tubing. Ensure it is firmly attached and not dangling. Inspect wiring for damage.
• 5. Reset and test: After thawing and correcting airflow issues, restore COOL mode. If E4 returns, the sensor may be out of range and require replacement.
• 6. If freezing persists: Low refrigerant or metering device issues are likely. Contact a licensed HVAC technician.