E4 Moderate Moderate DIY

MrCool Mini-Split E4 — Indoor Room Sensor (T1) Fault

MrCool Mini-Split · Error code E4

What E4 Means

E4 means the indoor room-temperature sensor (T1 thermistor) is reading open or shorted, so the unit can't measure room temperature.

Symptoms

  • Unit cycles oddly or stops
  • E4 on the display

Common Causes

1
Failed T1 thermistor

The room sensor has drifted out of range or failed.

2
Loose sensor connector

The thermistor plug on the indoor board is loose.

How to Fix It

  1. Power off and open the indoor head cover.
  2. Locate the T1 room sensor and reseat its connector on the control board.
  3. Test the thermistor resistance if you have a meter; replace the sensor if out of range.

Parts You May Need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

Common replacement Mini-Split Thermistor Sensor Kit View on Amazon→

More MrCool Mini-Split Codes

E1 E1 — Indoor/Outdoor Communication Error E2 E2 — Zero-Crossing Detection Error E3 E3 — Indoor Fan Speed Out of Control E5 E5 — Indoor Coil Sensor (T2) Fault F0 F0 — Overload / Current Protection F1 F1 — Outdoor Ambient Sensor (T4) Fault

All MrCool codes → All mini-split codes →

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.