Hello Kelly,under EOBD regs if the system detects a fault within so many drive cycles it will put the MIL on,But if the fault is what is called a "emission critical" fault under EOBD regs the MIL light must flash. emission critical" means that the fault if left untreated it may destroy the cat. Look at the system using EOBD instead of MEMS3 and it may tell you more. Regards Mark.