We present a new test generator for path delay faults in sequential circuits to generate validatable non-robust (VNR) tests. We use Boolean flags to generate VNR tests dynamically during the generation of robust tests, by relaxing certain off-path input requirements to those of non-robust tests. Results show that VNR tests provide a 10% improvement over the robust coverage of path delay faults in the sequential circuits considered. We adopt a 13-valued algebra to generate robust tests with hazards and non-robust tests. The algebra and implication tables eliminate the necessity to re-examine off-path inputs for a target path to determine the test validity. We provide examples to show that additional values at flip-flop inputs must be justified. This leads to identification of robust untestable faults without search. For the first time we present experimental results on robust and validatable non-robust test generation for ISCAS '89 sequential circuits in the non-scan mode using a variable clock scheme. Our test generator runs 26 times faster than previously published results for sequential circuits.