Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series error code 1 blink, 2.5 s OFF
Room temperature thermistor, or indoor outdoor communication error
Which models this applies to. This page covers the Mitsubishi Electric MR SLIM GL series split codes.
What is specific to the MSZ / MSY / MUZ / MUY GL NA series platform
MSZ and MSY indoor units with MUZ cooling only and MUY heating outdoor units on R410A. There is no numeric display: you count blinks of the upper operation indicator lamp on the indoor unit. The same blink count can mean an indoor fault or an outdoor one, and a three second steady hold before the blinking with no beep is what tells the two apart, with a second LED inside the outdoor unit narrowing several outdoor codes to one part.
Your control board does not print the code. It blinks an LED, and you count the flashes against a legend printed on the board or inside the access panel. Count a full cycle before you read it, because the pattern pauses and repeats.
Quick answer
On Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series, 1 blink, 2.5 s OFF is a fault. It means room temperature thermistor, or indoor outdoor communication error. A first thing to check: Indoor side: a failed room temperature thermistor, or a short or open in its wiring.
- 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
- Watch the lamp from the moment it comes on. Three seconds of steady light before any blinking means the outdoor unit is the problem; an immediate blink with a beep means the indoor unit is.
What 1 blink, 2.5 s OFF means on this model
Beep and an immediate blink means the indoor room temperature thermistor is short or open circuit, detected every eight seconds during operation. A three second steady lamp first with no beep means the outdoor unit cannot receive signals from the inverter board normally for three minutes, or a zero signal is being answered with a one signal thirty times in a row.
Symptoms you will notice
- Upper operation indicator lamp blinks once then pauses 2.5 seconds
- With a beep and no three second hold, the room reads wrong or the unit will not hold temperature
- With a three second steady lamp first and no beep, indoor and outdoor will not talk and the system will not run
- The whole system stops rather than just losing comfort
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 failed room temperature thermistor, or a short or open in its wiring
- Outdoor side: a communication wiring fault between indoor and outdoor
- Outdoor side: a serial signal error at the inverter board
- A loose or corroded communication connector
- Noise on the communication line from running it alongside mains wiring
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.
- Watch the lamp from the moment it comes on. Three seconds of steady light before any blinking means the outdoor unit is the problem; an immediate blink with a beep means the indoor unit is.
- Count at least two full blink cycles, which is what the manual asks for, because a single pattern is easy to miscount.
- Check the thermistor characteristics against the resistance the manual lists for the room temperature thermistor.
- Check the communication connectors at both units, and check that the communication cable was not run tight alongside the power cable.
Do not do these
- ×Do not start on the indoor unit if the lamp held steady for three seconds first. The fault is at the outdoor unit and the indoor thermistor is fine.
- ×Do not swap the room temperature thermistor on a three second hold fault. That part is not involved.
- ×Do not work on connectors with the power switch off and the supply isolated.
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.
Identifying which unit the fault is on is reasonable for a homeowner. Thermistor and board work is a technician job.
What it costs to fix
Parts
Room temperature thermistor $25 to $80. Communication cable $30 to $150. Inverter P.C. board $500 to $1,200.
Labor
One to three hours depending on which side the fault is on.
DIY or technician
Identifying which unit the fault is on 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
- How this model reports the code
- Blinking LED, counted against a legend
- Models in scope
- Mitsubishi Electric MSZ / MSY / MUZ / MUY GL NA series
- How this platform differs
- MSZ and MSY indoor units with MUZ cooling only and MUY heating outdoor units on R410A. There is no numeric display: you count blinks of the upper operation indicator lamp on the indoor unit. The same blink count can mean an indoor fault or an outdoor one, and a three second steady hold before the blinking with no beep is what tells the two apart, with a second LED inside the outdoor unit narrowing several outdoor codes to one part.
- 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.