Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series error code 3 blinks, 2.5 s OFF
Serial signal, or one of six outdoor thermistors
Quick answer
On Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series, 3 blinks, 2.5 s OFF is a fault. It means serial signal, or one of six outdoor thermistors. A first thing to check: Indoor side: a communication wiring fault or a serial signal error.
- Applies to
- Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series
- Mitsubishi Electric MR SLIM GL series split codes · Mini Split
- Type of code
- Fault
- Urgency: Needs attention soon
- First safe check
- Answer the three second hold question, then read the LED on the inverter P.C. board and count its blinks. That second number is what identifies the thermistor.
What 3 blinks, 2.5 s OFF means on this model
The most overloaded code on the platform. Beep and an immediate blink is a serial signal fault, where the signal from the outdoor unit is not received for up to six minutes. A three second hold is an outdoor thermistor short or open during compressor running, and the inverter board LED then says which one: once for the discharge temperature or defrost thermistor, twice for ambient, three times for fin, four times for the P.C. board, and no LED at all for the outdoor heat exchanger thermistor.
Symptoms you will notice
- Upper lamp blinks three times then pauses 2.5 seconds
- Beep and no hold means the indoor and outdoor are not communicating
- Three second hold means an outdoor thermistor has failed while the compressor runs
- The system trips shortly after the compressor starts
- The outdoor unit will not heat or cool at all
Common causes
The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:
- Indoor side: a communication wiring fault or a serial signal error
- Outdoor side: a failed discharge temperature thermistor or defrost thermistor
- Outdoor side: a failed ambient temperature thermistor
- Outdoor side: a failed fin temperature thermistor on the inverter board
- Outdoor side: a failed P.C. board temperature thermistor
- Outdoor side: a failed outdoor heat exchanger temperature thermistor
- A loose or damaged thermistor connector disturbed during service
What you can safely check
These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.
- Answer the three second hold question, then read the LED on the inverter P.C. board and count its blinks. That second number is what identifies the thermistor.
- Count at least two full cycles on the indoor lamp, as the manual instructs.
- Check the outdoor thermistors against the resistance characteristics the manual lists, and check the connectors at each one.
Do not do these
- ×Do not assume three blinks means the serial signal. Checking the three second hold takes seconds and is the difference between tracing a communication line and testing a thermistor.
- ×Do not replace thermistors one at a time until the inverter LED has told you which one. The manual makes six of them share this code.
- ×Do not work on the inverter board or its connectors with the supply live.
When to call a technician
Book a technician, do not keep resetting it
This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.
- The refrigerant circuit is sealed and legally requires licensed handling. Opening it without certification releases refrigerant and can put the whole system in violation of regulation, so a low-pressure or high-pressure fault on this equipment is a call-out, not a weekend job.
- If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.
Reading the two indicators is reasonable for a homeowner. Thermistor and board work is a technician job.
What it costs to fix
Parts
Outdoor thermistor $30 to $90. Inverter P.C. board $500 to $1,200. Communication cable $30 to $150.
Labor
Two to four hours once the thermistor is identified.
DIY or technician
Reading the two indicators is reasonable for a homeowner. Thermistor and board work is a technician job.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Mitsubishi Electric MR SLIM GL series split codes
- Models in scope
- Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.