Online performance characterization and faulty condition diagnosis of micro-combined-heat-and-power (μ-CHP) solid oxide fuel cell (SOFC) system is crucial for ensuring safe in-house operation and lifespan prolongation. However, fast accurate detection of faulty conditions and performance characterization of compact SOFC system remains a problem. This study applied multiple diagnostic methods to characterize a μ-CHP SOFC system, including chronopotentiometry, electrochemical impedance spectroscopy (EIS), total harmonic distortion (THD) tool. Performance characterization and condition diagnosis were performed under different power demands, fuel starvation, high-carbon fuel feed and long-term operation. EIS results under normal conditions and fuel starvation showed that the bottom 27-cell half stack, located away from the fuel inlet, received less fuel compared to the top 30-cell half stack. Dispersion analysis indicated the safe fuel utilization for the system should be maintained below 81 % to avoid fuel starvation. THD measurements revealed that fuel starvation could be effectively detected by sinusoidal excitation with frequencies between 0.01 and 0.1 Hz, where a high THD index was observed. The carbon deposition behavior was examined by adjusting the carbon to oxygen ratio (COR) at the catalytic partial oxidation reactor inlet. With COR up to 1, no significant carbon deposition was detected according to the EIS measurements, system voltage and temperature monitoring. During long-term operation under normal conditions, no substantial degradation was detected, demonstrating stable and reliable power and heat generation. The novelty of this work included (1) the identification of operational variability between two half stacks, (2) the rapid detection of fuel starvation conditions using THD analysis, (3) performance assessment under high carbon fuel feed, and (4) long-term degradation characterization.