Electrode carbon corrosion can occur from system start/stop cycles, hydrogen starvation, air storage, and vehicle driving cycles. Heavy duty fuel cell truck applications require high durability targets (20,000 – 50,000 hours), therefore, the state of health (SOH) investigation of carbon supported electrode becomes more important to understand the life of the electrode. Online CO₂ emission measurement and postmortem electrode thickness are common carbon SOH means. However, they are either resource intensive or destructive, and thus are not practical for long term durability test. In this work, several electrochemical diagnostics were used to study the changes in the cathode physical properties, kinetic activity, oxygen transport resistance, and proton transport resistance after controlled carbon corrosion tests. The diagnostics were compared at each stage of carbon SOH. We will discuss how carbon SOH can be estimated using a combination of these in-situ electrochemical methods alone.