ACiQ mini split error code E4

Common causes

• The indoor room temperature (T1) thermistor has failed (they do drift and fail with age and heat cycling).
• Its plug has worked loose from the control board, or a pin has corroded.
• The sensor lead is chafed, pinched or chewed, giving an intermittent open or short.
• The control board's sensor input has failed (rare compared with the sensor itself).

How to fix it yourself

Work through the checks below first; they resolve most cases. If they do not, the repair itself may need a technician.

1. Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
2. If the code comes straight back after a reset, the fault is real and not a glitch.
3. With power off, locate the indoor room temperature sensor (T1) just behind the front panel, on the air inlet side, usually held in a small clip. Follow its lead back to the board and reseat the plug firmly.
4. Inspect the lead for damage along its whole length, especially where it passes through the chassis.
5. If you have a multimeter, unplug the sensor and measure its resistance. A reading near zero ohms (short) or infinite (open) confirms a bad sensor. Most mini split thermistors read roughly 5 to 20 kΩ at room temperature, so a value in that range with a steady drift as you warm it in your hand means the sensor is fine and the board is the suspect.
6. Replacement thermistors are inexpensive and plug in. Match the part number from the parts list for your model.

Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.

When to call a technician

Sensor swaps are a doable DIY job on most units. Call a technician if the sensor tests good and the code persists, because that points at the control board.

Common causes

• A leaking flare joint (under- or over-torqued, or a scored flare face).
• Outdoor service valves not fully opened.
• A quick-connect coupling not fully seated.
• A kinked or crushed line set.
• A genuine leak on an older system.

How to fix it yourself

Work through the checks below first; they resolve most cases. If they do not, the repair itself may need a technician.

1. Open both outdoor service valves fully (counterclockwise until they stop).
2. Leak-check every connection with soapy water while the system runs. A stream of fine bubbles is a leak; re-torque to spec rather than cranking harder.
3. Inspect the line set for kinks, especially near the wall penetration.
4. Pull the front panel and check the filters and the indoor coil. Dirty filters and a matted coil restrict airflow, and airflow problems trigger a surprising number of protection codes.
5. Anything inside the refrigerant circuit (sensors are fine, but valves, compressors and leaks are not) needs EPA-certified refrigerant handling. That is a technician job.

Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.

When to call a technician

If a joint will not seal or you cannot find the leak, a technician must recover, repair, evacuate and recharge by weight.