The CH A-X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature Te=0.1–2.5 eV and electron density ne=(0.5–5)×1020 m-3] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to ≫106 due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of ∼100 for Te≪1 eV. The discrepancy is explained as the CH A level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for ne≫5×1020 m-3 and 0.1 eV≪Te≪1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.