Abstract: During active-standby switchover of Modicon Quantum CHS 110 hot standby system, some electric control valves may generate malfunctions. To solve the problem, test logics are added in PLC system programs in stations to detect variants with numerical changes to record their instantaneous values and corresponding time. Combined with the principles of the hot standby system, direct and root causes for malfunctions of relevant units are studied. Research results show that, during active-standby switchover of the Quantum hot standby system, direct cause for malfunction of the control valve is the initialization of relevant variants, whereas the root cause is the prolonged data transmission among hot standby modules and relevant data losses due to improper allocation of memories in programming of the PLC system. To solve these problems, memory allocation and software programming of the PLC system should be modified.