Della Mini Split E4 Troubleshooting

That dreaded E4 code… it means the AC cooling system detected a problem with refrigerant flow. The unit stops cooling and may not run normally until the issue is resolved. Turn it off at the breaker. Do not keep running it.

Work through the steps below in order. Stop as soon as E4 clears.

What Does E4 Mean?

E4 stands for "AC Cooling System Abnormal (Gas Not Enough)" in the service manual. The system detected that refrigerant is not circulating the way it should.

It can be caused by:

• Service valves left closed after installation or a recent maintenance visit
• Low refrigerant charge
• A blockage inside the refrigerant circuit
• A failed temperature sensor inside the indoor or outdoor unit
• A failed control board

The steps below start with what you can check yourself, then move to what needs a technician.

Step 1: Check the Service Valves

Before you call anyone, try this. It takes about two minutes and no tools.

The outdoor unit has two service valves where the copper refrigerant lines connect. These valves are sometimes left closed after installation or a service visit. If they are closed, refrigerant cannot flow, and that can trigger an E4 even when the system is fully charged.

Find the two brass ports on the side of the outdoor unit. Each has a cap. Remove both caps. Turn each valve stem counterclockwise until it stops. This fully opens both valves and restores normal refrigerant flow.

Restore power, turn the unit on, and check the display:

• If E4 is gone, a closed valve was the problem. Done.
• If E4 is still showing, the valves were already open and the pressure is genuinely low. Continue to Step 2.

Step 2: Have a Technician Check the Refrigerant Pressure

Checking refrigerant pressure isn’t a DIY job. You need a gauge set and a licensed HVAC tech. When you call, tell them the unit is showing E4 and that both service valves are confirmed open.

On a Della mini split, normal pressure runs 116–189 PSI in cooling mode or 290–522 PSI in heating mode. The technician will verify this with gauges.

If the reading is in range, refrigerant isn’t the issue. Skip to Step 3.

If pressure is low, the technician will add refrigerant. After recharging, restore power and check the display:

• If E4 is gone, low refrigerant was the cause. Done.
• If E4 is still showing after the recharge, there is a blockage in the refrigerant circuit. The technician will need to inspect and service the expansion valve and refrigerant lines. This is not something to troubleshoot further on your own.

Step 3: Test the Temperature Sensors

If Step 2 shows refrigerant pressure is normal, check the three temperature sensors next. A failed or internally shorted sensor can send bad data to the control board. That can trigger an E4 even when refrigerant is fine.

The three sensors are:

• Indoor coil sensor (IPT): a small probe clipped to the evaporator coil inside the indoor unit. At 77 degrees F it should read about 5,000 ohms on a multimeter.
• Outdoor coil sensor (OPT): a probe clipped to the condenser coil inside the outdoor unit. At 77 degrees F it should also read about 5,000 ohms.
• Discharge temperature sensor (DTS): a probe on the discharge pipe near the compressor, inside the outdoor unit. At 77 degrees F it should read about 20,000 ohms.

Sensor positions vary between models. Resistance values shift with temperature, so the numbers above are reference points at 77 degrees F only.

How to test each sensor:

1. Turn off the unit and cut power at the breaker. Wait a few minutes before opening any panels.
2. Locate the sensor. It is a small probe clipped to a coil or pipe, with a wire that runs back to the control board. Unplug that wire connector from the board.
3. Set a multimeter to resistance (ohms) mode. Touch one probe to each terminal on the sensor connector.
4. A reading of 0 means the sensor is shorted. A reading of OL or infinity means it is broken. Either result means the sensor needs to be replaced.

Test all three sensors:

• Any sensor reads 0 or OL: replace it. Restore power and check the display. If E4 is gone, troubleshooting is complete.
• All three sensors read in the normal range: the sensors are not the problem. Move to Step 4.

Step 4: Replace the Control Boards

If you’ve reached this step, pressure is correct and sensors are fine. That leaves the control board. It reads the sensors and runs the system. There is one in the indoor unit and one in the outdoor unit.

Have a technician replace the indoor unit control board first. Restore power and check the display:

• If E4 is gone, the indoor board was the cause. Done.
• If E4 is still showing, have the technician replace the outdoor unit control board.

Boards replaced? That’s the end of the line. If E4 is still showing, call Della support. Warranty may cover it.

Quick Reference

Work through these in order and stop when E4 clears:

1. Check the two service valves on the outdoor unit and open them if closed (turn counterclockwise until it stops)
2. If E4 remains, have a technician check refrigerant pressure (normal: 116 to 189 PSI cooling / 290 to 522 PSI heating)
3. If pressure is low, technician adds refrigerant; if E4 remains after recharge, the refrigerant circuit needs servicing
4. If pressure is correct, test the three sensors (indoor coil, outdoor coil, discharge) with a multimeter; replace any that read 0 or OL
5. If all sensors are fine, replace the indoor control board first; if E4 remains, replace the outdoor control board

Key Notes

• Sensor replacement and control board replacement are separate paths. If a sensor is faulty, replace it and troubleshooting is complete. You only move to the control boards if all sensors test normal.
• When replacing control boards, do the indoor unit first.
• Restore power and check the display after each fix before moving to the next step.

When to Call a Professional

This is licensed HVAC work: refrigerant pressure checks, recharging, and control board replacement. If your AC is acting dramatic, this is where things get serious. All of it requires a technician. Homeowners can only do two checks: service valves and sensor resistance with basic tools.

For parts, warranty questions, or to reach Della's service team, visit the Della support center.