A control flow fault occurs when a processor fetches and executes an incorrect next instruction. Executable assertions, i.e., special instructions that check some invariant properties of a program, provide a powerful and low-cost method for on-line detection of hardware-induced control flow faults. We propose a technique called ACFC (Assertions for Control Flow Checking) that assigns an execution parity to a basic block, and uses the parity bit to detect faults. Using a graph model of a program, we classify control flow faults into skip, re-execute and multi-path faults. We derive some necessary conditions for these faults to manifest themselves as execution parity errors. To force a control flow fault to excite a parity error, the target program is instrumented with additional instructions. Special assertions are inserted to detect such parity errors. We have a developed a preprocessor that takes a C program as input and inserts ACFC assertions automatically. We have implemented a software-based fault injection tool SFIG which takes advantage of the GNU debugger. Fault injection experiments show that ACFC incurs less performance overhead (around 47%) and memory overhead (around 30%) than previous techniques, with no significant loss in fault coverage.