Fault codes

HVAC fault code library

Search verified HVAC fault-code references by code, brand, or symptom. Start with the documented meaning and field checks, then verify the exact model and controller service literature before acting.

Previewing a few public fault codes

Create a Free Technician account or sign in to search all fault codes and detailed diagnostic checks. No paid subscription required.

Create free account Sign in

4 results

U4-01

Daikin · VRV / VRF (verify exact model)

Caution

Indoor/outdoor transmission wiring fault (model-specific reference)

Current Daikin VRV installation/service literature lists U4-01 as an indoor/outdoor transmission wiring fault. Exact terminals, topology, sub-code behavior, and reset steps vary by product family and generation.

Things to check

Open, shorted, or incorrectly landed indoor/outdoor transmission wiring
Loss of power to a required indoor or outdoor device
A connected device or control board disrupting normal transmission

Start here: Record the complete indoor/outdoor model numbers and full code, then use the matching Daikin service literature. Confirm required equipment power and inspect the specified transmission wiring/topology before condemning a board or changing network wiring.

Model note: Daikin VRV installation literature lists U4-01 as faulty indoor/outdoor transmission wiring. Use the manual for the exact installed product because terminals and troubleshooting steps vary by system.

Open full reference Manufacturer source

2502

Mitsubishi Electric · CITY MULTI / indoor unit (verify model)

Caution

Drain pump abnormality (model-specific CITY MULTI reference)

A Mitsubishi Electric CITY MULTI service handbook lists checking code 2502 as a drain-pump abnormality. Detection logic, connectors, resistance values, and applicable components vary by indoor-unit model and generation.

Things to check

Drain water level remains high because the drain pump/lift path is not removing water
Drain-sensor/heater wiring or connector problem on models that use that detection method
Indoor control-board detection fault after the drain pump and sensor circuit are proven

Start here: Record the complete indoor-unit model and controller code, then use the matching service manual. Inspect the pan and discharge path, prove drain-pump operation, check the sensor/connector circuit by the model procedure, and only then condemn the indoor board or order model-specific pump/hose parts.

Model note: Mitsubishi Electric CITY MULTI service literature lists checking code 2502 as a drain-pump abnormality. Use the manual for the exact indoor-unit model because detection details and component values differ.

Open full reference Manufacturer source

Daikin · VRV / VRF (verify exact model)

Caution

Refrigerant shortage

Daikin VRV IV-S service literature lists U0 as refrigerant shortage. Treat it as a starting point for model-specific refrigerant diagnosis, not as permission to add charge by pressure.

Things to check

Refrigerant leak or insufficient refrigerant mass
Charge quantity does not match the installed piping/application
A refrigeration or sensing condition that must be checked against the exact model service procedure

Start here: Verify airflow/load and record the complete model and operating data. Leak-search before adding refrigerant, then repair, pressure-test, evacuate, and charge by the exact Daikin procedure and documented quantity/charging method.

Model note: Daikin VRV IV-S service literature lists U0 as refrigerant shortage. Detection logic and charging procedure vary by VRV family and model.

Open full reference Manufacturer source

1302

Mitsubishi Electric · CITY MULTI (verify exact model)

Critical

High pressure abnormality

Mitsubishi Electric CITY MULTI service literature lists 1302 as a high-pressure abnormality on applicable outdoor units.

Things to check

Poor condenser heat rejection, including fan/airflow or water-flow problems
Refrigerant-side condition such as excess charge, restriction, or abnormal system control
High-pressure sensor/switch, connector, wiring, or input-circuit problem

Start here: Record the exact outdoor-unit model and operating mode. Verify real system pressure with appropriate instruments, prove condenser heat rejection, compare sensor values with actual pressure, and follow the model-specific 1302 diagnostic tree before resetting repeatedly.

Model note: The pressure thresholds and diagnostic branches differ by CITY MULTI generation and model.

Open full reference Manufacturer source