Question 1

What does Mitsubishi error code U4 mean?

Accepted Answer

Mitsubishi U4 is the most common error code and indicates a communication error between the indoor and outdoor units. The most frequent cause is loose wire nut connections on the communication wiring. Re-terminate all wire connections, check for damaged wiring, and power cycle both units. If the error persists, the control board may need replacement.

Question 2

What does Mitsubishi error code E6 mean?

Accepted Answer

Mitsubishi E6 indicates an indoor/outdoor communication error. Check the communication wiring between units (S1/S2 terminals), verify all wire connections are tight at both boards, and check for a 24V signal between the units. This is often a wiring issue rather than a board failure.

Question 3

What does Mitsubishi error code P1 mean?

Accepted Answer

Mitsubishi P1 indicates an intake (room temperature) sensor error. The thermistor is either open circuit, short circuit, or has a bad connector. Measure the thermistor resistance, which should be approximately 10k ohms at 77 degrees F. Replace the thermistor if it is out of spec.

Question 4

How do I read Mitsubishi mini-split error codes?

Accepted Answer

Mitsubishi mini-splits display alphanumeric error codes on the indoor unit display or via LED flashes. E-series codes indicate communication errors, P-series codes indicate sensor faults, and U-series codes indicate system protection or communication issues. U4 (communication error) is by far the most common code technicians encounter.

Code	Description	Common Cause	Fix
E6	Indoor/outdoor communication error	Wiring issue between units, bad control board, or loose connection	Check communication wiring (S1/S2), verify wire connections at both boards, check for 24V signal between units
E9	Indoor/outdoor comm error (duplex)	Same as E6 but in multi-zone systems	Check branch box wiring, verify address settings
P1	Intake sensor error	Open or short in intake thermistor, or bad connector	Measure thermistor resistance (10k ohm at 77°F), check connector
P2	Pipe (coil) sensor error	Open or short in pipe thermistor, or poor contact with pipe	Check thermistor resistance, ensure good thermal contact with pipe
P4	Drain sensor error	Drain pan full, clogged drain, or failed float switch	Clear condensate drain, clean drain pan, check float switch
P6	Freeze/overheat protection	Low airflow, dirty filter, low charge, or blocked coil	Clean filters, check airflow, check refrigerant charge
U1	High pressure trip	Dirty outdoor coil, failed outdoor fan, overcharge, or non-condensables	Clean outdoor coil, check outdoor fan, verify charge
U2	Compressor overcurrent / low voltage	Low voltage supply, failing compressor, or bad capacitor	Check supply voltage, measure compressor amps, test capacitor
U4	Communication error (MOST COMMON)	Loose wire nuts, damaged comm wire, board failure, or power surge	Re-terminate all wire connections, check for damaged wiring, power cycle both units, replace board if damaged

Mitsubishi Error Codes & Fault Codes

Detailed Code Reference

Frequently Asked Questions

What does Mitsubishi error code U4 mean?

What does Mitsubishi error code E6 mean?

What does Mitsubishi error code P1 mean?

How do I read Mitsubishi mini-split error codes?

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