This paper presents a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses the Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. The authors compare APEC results with other plasma codes for hydrogen- and helium-like diagnostics and test the sensitivity of their results to the number of levels included in the models. They find that dielectronic recombination with hydrogen-like ions into high principal quantum numbers $(n = 6 - 10)$ affects some helium-like line ratios from low-lying $(n = 2)$ transitions. The study focuses on two line ratios: the H-like Ly$\beta$/Ly$\alpha$ ratio and the He-like G ratio. The results show that near the peak emissivity of the emission lines, the code predictions agree within 25%, but substantial disagreements exist even for these simple ions when they are not near peak emissivity. The Bethe limit is used to check the agreement with the APEC result, which is reassuring. The paper also discusses the impact of different atomic data on the observed line ratios and the importance of careful treatment of recombination to individual levels and cascades for diagnostic line ratios.This paper presents a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses the Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. The authors compare APEC results with other plasma codes for hydrogen- and helium-like diagnostics and test the sensitivity of their results to the number of levels included in the models. They find that dielectronic recombination with hydrogen-like ions into high principal quantum numbers $(n = 6 - 10)$ affects some helium-like line ratios from low-lying $(n = 2)$ transitions. The study focuses on two line ratios: the H-like Ly$\beta$/Ly$\alpha$ ratio and the He-like G ratio. The results show that near the peak emissivity of the emission lines, the code predictions agree within 25%, but substantial disagreements exist even for these simple ions when they are not near peak emissivity. The Bethe limit is used to check the agreement with the APEC result, which is reassuring. The paper also discusses the impact of different atomic data on the observed line ratios and the importance of careful treatment of recombination to individual levels and cascades for diagnostic line ratios.