Diagnostic diagrams of the [Cii]/[Oi]63 ratio plotted against the ([Cii]+[Oi]63)/F_TIR ratio (left panels) and the [Cii]/[Oi]145 versus ([Cii]+[Oi]145)/F_TIR ratio (right panels) for NGC 891. We superimpose our adjusted observations onto a grid of constant hydrogen nuclei density, log n (black dashed lines), and FUV radiation field strength, log G₀ (red solid lines), determined from the Kaufman et al. (1999, 2006) PDR model. Each data point represents one pixel, with colours as described in Fig. 11. We present our unadjusted observations (upper panels) and the observations including the adjustments applied to the [Cii], [Oi] 63 and F_TIR emission as described in Sect. 4.1. We compare our two approaches to estimate (and remove) the fraction of the [Cii] emission arising from ionised gas, whereby one method uses the reliable measurements of the [Nii] 205 μm line emission (middle panels) and the alternative method uses the 24 μm emission as a proxy for the [Nii] 205 μm line emission (lower panels) via the correlation presented in Fig. 8. Error bars do not account for the uncertainties in these corrections to the [Cii] emission.