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
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
- 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.
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
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.