Mitsubishi Mr Slim U2 Error — Discharge Temperature Fix

What it means

Mitsubishi Electric Mr Slim U2 is the abnormal high discharge temperature error documented on Mitsubishi's Mr Slim error code list.
The official description: 'abnormal high discharging temperature / 49C worked / insufficient refrigerant.'
The 49C is a compressor inner thermostat (Klixon) — when the compressor discharge gas temperature exceeds the design limit, the 49C trips and the system shuts down on U2.
The most common cause is refrigerant shortage; the compressor runs hotter when there isn't enough refrigerant to cool itself between cycles.

Affected Models

Mitsubishi Mr Slim P-Series outdoor units (PUZ, PUY, PUHZ series)
Mitsubishi Mr Slim K-Series outdoor units (older lineup)
Mitsubishi City Multi PURY and PUMY outdoor heat pump models
U2 appears on the wall remote with no specific indoor unit detail — it's an outdoor-side fault
Mitsubishi's documentation: U2 = abnormal high discharge temp / 49C worked / insufficient refrigerant

Common Causes

Refrigerant shortage — most common cause (slow leak over years)
Stop valves at the outdoor unit not fully open after install
Discharge thermistor (TH4) faulty or its connector loose
LEV (linear expansion valve) connector disconnected or stuck
Compressor itself overheating from age or oil loss

How to Fix It

Stop running the AC.

Turn the unit off at the remote.
Don't keep cycling power.
The 49C internal thermostat is protecting the compressor — repeated attempts to restart while hot accelerate wear.
Leave it off until a technician can attend.
Confirm the outdoor stop valves are fully open.

If U2 appeared right after install or service, the most likely cause is closed stop valves.
At the outdoor unit, look for two service valve caps (large and small refrigerant lines).
Remove the caps.
The valve stem inside should be fully out (back-seated).
If a valve is closed (stem fully in) or partially closed, that's the cause — open it fully.
This is a quick visual check but if you're unsure, leave it to the technician.
Check for visible signs of refrigerant leak.

While waiting for service, walk the line set between the indoor and outdoor unit.
Oil staining at flare connections, the indoor coil, or along the lineset points to refrigerant leak.
Note where any oil is and tell the technician — it saves diagnostic time.
Note the recent service history.

Was the system installed or serviced recently?
U2 right after install = often closed stop valves.
U2 a few months after service = a leaking flare connection from the work.
U2 with no recent service = slow leak over years or compressor age.
Telling the technician the history changes the diagnostic approach.
Schedule HVAC service with Mitsubishi capability.

U2 requires manifold gauges to verify the cause.
The technician will check operating pressures (high and low side), measure discharge temperature with a clamp thermometer on the discharge line, and verify the TH4 thermistor and LEV connectors.
Schedule through Mitsubishi Electric service or your installer.
Expect leak repair, evacuation, and recharge by weight.

If U2 was caused by refrigerant shortage, the fix is: pressure-test the system to find the leak, repair the leak (re-flare, re-braze, or replace the leaking section), pull a vacuum, recharge with the correct R-410A or R-32 charge by weight per the unit's nameplate.
This is a several-hour visit, not a quick top-up.
A top-up without leak fix means U2 returns in months.

When to Call a Professional

U2 needs an HVAC technician with refrigerant capability.
The 49C internal thermostat won't allow restart until the compressor cools — but the underlying cause (low refrigerant or stuck expansion valve) must be fixed first, otherwise U2 fires again within minutes.
Continuing to retry without fixing the cause overheats the compressor permanently.

Frequently Asked Questions

Can a Mitsubishi U2 damage the compressor permanently?

Repeated U2 events can shorten compressor life significantly.
The 49C internal thermostat is designed to catch transient overheating events and protect the compressor — a single trip is fine.
But a system run with chronically low refrigerant has the compressor running hot continuously between trips, which dries out the compressor oil and eventually seizes the bearings.
The protection isn't a substitute for fixing the underlying refrigerant shortage.
If you've had U2 fire repeatedly over weeks without fixing the cause, the compressor itself may now be damaged — get service before the cost goes from 'leak fix' to 'leak fix + compressor replacement'.