By comparing of the spectra from the Chandra LETGs observation with the synthetic spectra of argon, we identify two weak emission lines, that is, the Ar XV 27.470A line with transition 2s3p 1P1 - 2s2 1S0 and the Ar XVI 27.872 A line with transition 3d 2D3/2 - 2p 2P1/2, which blend with the N Ⅶ Lyα emission line. Several secondorder spectral lines are also identified, one of which is the second-order Fe XⅧ 14.534A line with transition 2p4(3P)3d 2F5/2 - 2p5 2Ps/2, which blends with the inter-combination line of He-like N Ⅵ. In order to study the effects of these new identified weak lines on the electron temperature and density, we diagnose the two parameters using the intensity ratio of triplets of the He-like N Ⅵ and that of resonance lines of H- and He-like N. In the literature, there is a discrepancy of the temperatures deduced from N Ⅵ from other He-like ions, such as C Vand O Ⅶ. The discrepancy disappears after the contribution of these weak lines is taken into account.