Fujitsu Error Code E4:00: Indoor Coil Sensor Open
How to Fix Fujitsu Code E4:00
- Step 1: Turn the unit off and then back on. Sometimes a temporary glitch can trigger the error code. Wait a few minutes after turning it off before turning it back on.
- Step 2: Check the area around the indoor unit for any obvious obstructions blocking airflow. Make sure the air filter is clean.
- Step 3: If the error persists after restarting the unit, contact a qualified HVAC technician to diagnose and repair the problem.
How to Reset Fujitsu Code E4:00
- Turn off the unit using the remote control or wall controller.
- Turn off the circuit breaker for the outdoor and indoor units. Wait 60 seconds.
- Restore power at the breaker.
- Turn the unit on with the remote control. If the error returns, the fault needs professional diagnosis.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Fujitsu Error Code E4:00 Mean?
The Fujitsu E4:00 error code indicates an open circuit in the indoor coil temperature sensor. This sensor, also called a thermistor, is responsible for measuring the temperature of the indoor coil (the part that gets cold when cooling or hot when heating). The sensor sends this temperature information back to the unit's control board, which uses it to regulate the cooling or heating process. An 'open' circuit means the electrical connection is broken, so the control board isn't receiving any temperature data. Because the control board doesn't know the coil temperature, the unit will likely stop operating or run inefficiently to protect itself from damage (like freezing the coil). This prevents the unit from properly heating or cooling your space.
Common Causes
- Faulty indoor coil temperature sensor (thermistor)
- Loose or disconnected wiring at the sensor or control board
- Corrosion or damage to the sensor wiring or connector
- Defective indoor unit control board
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Unit displays E4:00 error | Measure resistance across the indoor coil temperature sensor terminals at the control board with the unit powered down and wires disconnected. | Resistance should match the sensor's temperature/resistance chart (typically around 5-15k ohms at room temperature). An open circuit (infinite resistance) indicates a faulty sensor or wiring. |
| Sensor resistance is within range | Inspect the wiring harness and connectors between the sensor and the control board for damage, corrosion, or loose connections. | Wiring and connectors should be clean, secure, and free from damage. |
| Wiring and sensor appear good | Check the voltage at the sensor input on the control board. | Typically 5V DC. Absence of voltage indicates a faulty control board. |
Applicable Fujitsu Models
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Indoor Coil Temperature Sensor | — | — | — |
| Indoor Unit Control Board | — | — | — |
Related Codes on Same Models
Other codes that affect the same Fujitsu model families as E4:00.
| Code | Description | Severity |
|---|---|---|
| E0:00 | Communication Error (Indoor to Remote) | Warning |
| E1:00 | Communication Error (Indoor to Outdoor) | Warning |
| E2:00 | Room Temp Sensor Open Circuit | Warning |
| E3:00 | Room Temp Sensor Short Circuit | Warning |
| E5:00 | Indoor Coil Sensor Short | Warning |
| E6:00 | Outdoor Coil Sensor Fault | Warning |
| E7:00 | Outdoor Ambient Sensor Fault | Warning |
| E8:00 | Indoor Fan Motor Fault | Warning |
| E9:00 | Drain Pump / Float Switch | Warning |
| EE:00 | Communication Error (Multi-System) | Warning |