Defective microfluidic chip increases the assay completion and total turnaround time and it is the main reason of deviation of the actual result of assay operation. In online testing, the test droplets are moving out of step with each other. Again testing of DMFB is NP-hard in nature. Using Euler path based test technique, we can test the whole chip but we cannot apply Hamiltonian path based test method in case of equilateral triangular electrode array due to routing constraints in triangular DMFB. A graph-based test planning technique for online testing is proposed in this paper. Considering the active parts of the chip, like mixer and store cell, as obstacle in the triangular microfluidic array, we test the chip during the assay operation running in some portions of the array. Here, we focus on Euler path based test technique for catastrophic fault detection. © 2018, Springer Nature Singapore Pte Ltd.