MrCool · Mini-Split Moderate

E4 — Indoor Room Sensor (T1) Fault

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

Keep using it?
Yes, with caution
DIY difficulty
Hard
Typical cost
$15-30
Verified against
support.mrcool.com ↗

The fix

MrCool indoor unit temperature sensor/thermistor

Indoor room temperature sensor (T1 thermistor) open or shorted

$15-30 moderate difficulty 30-45 min Tools: screwdriver, multimeter

Diagnose first: MrCool publishes an official DIY temperature-sensor replacement guide for this exact fault — check the connector for a loose pin before ordering a new sensor.

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How to fix it

  1. Power-cycle at the breaker: off for five minutes, back on. Sensor codes occasionally clear if the real problem was a momentary bad reading.
  2. If E4 returns, call an HVAC technician. The sensor lives inside the indoor unit, and testing it means opening the head and measuring it — not owner-safe work.
  3. What the technician will do: test the T1 sensor against its resistance specification and replace it if it has failed — one of the quicker, cheaper mini-split repairs. Meanwhile the unit may run with rough temperature control, which is annoying but not harmful.

Why it happens

Typical signs: Unit cycles oddly or stops · E4 on the display

  • Failed T1 thermistor The room sensor has drifted out of range or failed.
  • Loose sensor connector The thermistor plug on the indoor board is loose.

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 →

Frequently asked questions

What does the E4 code mean on a MrCool mini-split?

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

What causes the E4 code on a MrCool mini-split?

Failed T1 thermistor: The room sensor has drifted out of range or failed. Loose sensor connector: The thermistor plug on the indoor board is loose.

Is it safe to keep using a MrCool mini-split with a E4 error?

Yes, with caution. This is rated a moderate-severity fault.

How do I fix E4 on a MrCool mini-split?

Power-cycle at the breaker: off for five minutes, back on. Sensor codes occasionally clear if the real problem was a momentary bad reading. If E4 returns, call an HVAC technician. The sensor lives inside the indoor unit, and testing it means opening the head and measuring it — not owner-safe work. What the technician will do: test the T1 sensor against its resistance specification and replace it if it has failed — one of the quicker, cheaper mini-split repairs. Meanwhile the unit may run with rough temperature control, which is annoying but not harmful.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: support.mrcool.com.