Diagnosing Air Conditioner F3 Fault Code

Fault description

One Daikin 5P air conditioner in a server room repeatedly tripped with the F3 fault code and shut down. After power-off and restart it would run and provide good cooling, but after some time it would stop again and the wired remote showed F3.

Manufacturer fault code meaning

According to reference material, Daikin F3 indicates an abnormal discharge pipe temperature. Possible causes include: discharge pipe thermistor failure; compressor surface thermistor failure; compressor surface temperature abnormality; insufficient refrigerant; blocked piping; connector contact issues; or other faults that cause elevated discharge temperature. Using a multimeter, the discharge pipe thermistor and compressor surface thermistor were checked and found to be normal. This suggests the actual discharge pipe or compressor surface temperature was rising, so the next step was to find the reason for overheating.

Potential causes considered

Common reasons for overheating include:

Compressor failure
Contamination or blockage in the refrigeration system
Dirty outdoor unit heat exchanger fins causing poor heat dissipation and high discharge pressure
Insufficient airflow due to fan failure or degraded fan start capacitor

Because cooling performance remained good, compressor failure and refrigeration system blockage were considered unlikely. The outdoor unit heat exchanger fins were inspected and were not dirty, so poor fin heat dissipation was also excluded.

Diagnosis and repair

Long-term observation found that one of the outdoor fans intermittently stopped. When that fan stopped, the compressor also stopped and the wired remote displayed F3. This indicates the F3 fault was caused by reduced airflow from the stopped fan, leading to inadequate heat dissipation, elevated discharge pipe temperature, and a protective shutdown with F3 code.

The fan would run at startup and then stop, suggesting the fan motor itself was likely fine and the issue was in the outdoor unit control PCB. The outdoor control board was removed and inspected. The fan circuit contains three relays to select low, medium, and high fan speeds. One relay output contact was open. The relay was replaced and the control board reinstalled. After the relay was replaced, the air conditioner no longer produced the F3 fault and did not shut down.